chore: decrease package size

[ehmicky]

This decrease the npm package size, from 53KB to 6.7KB.

Before:

npm pack --dry
npm notice 
npm notice 📦  @netlify/[email protected]
npm notice === Tarball Contents === 
npm notice 2.0kB  CHANGELOG.md                                                               
npm notice 5.5kB  CODE_OF_CONDUCT.md                                                         
npm notice 1.1kB  LICENSE.md                                                                 
npm notice 6.6kB  README.md                                                                  
npm notice 606B   .github/ISSUE_TEMPLATE/bug_report.md                                       
npm notice 595B   .github/ISSUE_TEMPLATE/feature_request.md                                  
npm notice 822B   .github/workflows/fossa.yml                                                
npm notice 655B   .github/workflows/test.yml                                                 
npm notice 655B   .github/workflows/test.yml~                                                
npm notice 235B   .prettierignore                                                            
npm notice 47B    .prettierrc.json                                                           
npm notice 697B   helpers/doesNotNeedPlugin.js                                               
npm notice 888B   helpers/doesSiteUseNextOnNetlify.js                                        
npm notice 1.1kB  helpers/hasCorrectNextConfig.js                                            
npm notice 651B   helpers/isStaticExportProject.js                                           
npm notice 577B   helpers/isStaticExportProject.js~                                          
npm notice 1.4kB  helpers/validateNextUsage.js                                               
npm notice 1.4kB  helpers/validateNextUsage.js~                                              
npm notice 1.9kB  index.js                                                                   
npm notice 3.5kB  index.js~                                                                  
npm notice 28B    manifest.yml                                                               
npm notice 38.2kB next-on-netlify.png                                                        
npm notice 1.4kB  package.json                                                               
npm notice 1.5kB  package.json~                                                              
npm notice 0B     test/fixtures/deep_invalid_next_config/base/.gitkeep                       
npm notice 0B     test/fixtures/functions_copy_files/out_functions/next_random/next_random.js
npm notice 0B     test/fixtures/functions_copy_files/out_functions/next_random/nextJsPage.js 
npm notice 0B     test/fixtures/publish_copy_files/out_publish/subdir/dummy.txt              
npm notice 6.2kB  test/index.js                                                              
npm notice 5.4kB  test/index.js~                                                             
npm notice 100B   test/sample/netlify.toml                                                   
npm notice 98B    test/sample/netlify.toml~                                                  
npm notice 315B   test/sample/package.json~                                                  
npm notice 105B   test/sample/pages/_app.js                                                  
npm notice 140B   test/sample/pages/api/test.js                                              
npm notice 100B   test/sample/pages/index.js                                                 
npm notice 706B   test/sample/TESTING.md                                                     
npm notice === Tarball Details === 
npm notice name:          @netlify/plugin-nextjs                  
npm notice version:       2.0.0                                   
npm notice filename:      @netlify/plugin-nextjs-2.0.0.tgz        
npm notice package size:  53.0 kB                                 
npm notice unpacked size: 85.1 kB                                 
npm notice shasum:        0c71df74a7d42398f798a1fc3a0b0ee2120d7b4a
npm notice integrity:     sha512-eXnMM1CvBaQm4[...]7LXQBo59iXgGA==
npm notice total files:   37                                      
npm notice 
netlify-plugin-nextjs-2.0.0.tgz

After:

npm pack --dry
npm notice 
npm notice 📦  @netlify/[email protected]
npm notice === Tarball Contents === 
npm notice 2.0kB CHANGELOG.md                       
npm notice 1.1kB LICENSE.md                         
npm notice 6.6kB README.md                          
npm notice 697B  helpers/doesNotNeedPlugin.js       
npm notice 888B  helpers/doesSiteUseNextOnNetlify.js
npm notice 1.1kB helpers/hasCorrectNextConfig.js    
npm notice 651B  helpers/isStaticExportProject.js   
npm notice 1.4kB helpers/validateNextUsage.js       
npm notice 1.9kB index.js                           
npm notice 28B   manifest.yml                       
npm notice 1.5kB package.json                       
npm notice === Tarball Details === 
npm notice name:          @netlify/plugin-nextjs                  
npm notice version:       2.0.0                                   
npm notice filename:      @netlify/plugin-nextjs-2.0.0.tgz        
npm notice package size:  6.7 kB                                  
npm notice unpacked size: 17.8 kB                                 
npm notice shasum:        5cebfe2175d5120ab5ca4733ed522598819e6eb1
npm notice integrity:     sha512-YlgBzfwV8Vnd4[...]wnOOXa3y7URow==
npm notice total files:   11                                      
npm notice 
netlify-plugin-nextjs-2.0.0.tgz

[lindsaylevine]

oh wow awesome 📝 thank you for this!!!!!

[ehmicky]

Note: we will need to remember to update the files property in package.json if we add any new files outside of helpers/**/*.js and index.js.