feat: convert project name to lowercase and warn on creation

closes vuejs#2547
closes vuejs#5032

I'm still very hesitant on adding this feature, though.

First, this change allows project creation in a folder with uppercase
letters in its name. It is strongly discouraged and may cause many
weird issues all over the ecosystem.

For example, vuejs#5022, vuejs#4424, vuejs#3665, vuejs#4174#issuecomment-569709494 are all
caused by case issues. Adding support for uppercase project names will
only worsen this situation.

Secondly, it adds a lot of maintenance burden to us. As noted in the
comments, these prompts are hard to test right now (because
`createTestProject` runs in another process so it's hard to intercept
the prompts). Even if such test utilities are added in the future, it's
still very tedious to take care of all the case issues in the test
suite.

What's worse is that we can affect the project folders created by
@vue/cli by converting the project name to lower case. But for
`vue create .`, we cannot change the current folder's name. So, we'll
have another edge case to test.

Author: haoqunjiang

packages/@vue/cli/lib/create.js

@@ -7,16 +7,19 @@ const { getPromptModules } = require('./util/createTools')
 const { chalk, error, stopSpinner, exit } = require('@vue/cli-shared-utils')
 const validateProjectName = require('validate-npm-package-name')
 
+// TODO: add test cases for prompts in this file
 async function create (projectName, options) {
   if (options.proxy) {
     process.env.HTTP_PROXY = options.proxy
   }
 
   const cwd = options.cwd || process.cwd()
   const inCurrent = projectName === '.'
-  const name = inCurrent ? path.relative('../', cwd) : projectName
-  const targetDir = path.resolve(cwd, projectName || '.')
 
+  const originalName = inCurrent ? path.relative('../', cwd) : projectName
+  const name = originalName.toLowerCase()
+
+  // TODO: should also implement this logic in the UI
   const result = validateProjectName(name)
   if (!result.validForNewPackages) {
     console.error(chalk.red(`Invalid project name: "${name}"`))
@@ -29,6 +32,21 @@ async function create (projectName, options) {
     exit(1)
   }
 
+  if (name !== originalName) {
+    const { continueWithLowerCase } = await inquirer.prompt([
+      {
+        name: 'continueWithLowerCase',
+        type: 'confirm',
+        message: `Uppercase is not allowed in an npm package. Continue with the name ${chalk.yellow(name)}?`
+      }
+    ])
+
+    if (!continueWithLowerCase) {
+      return
+    }
+  }
+
+  const targetDir = path.resolve(cwd, inCurrent ? '.' : name)
   if (fs.existsSync(targetDir) && !options.merge) {
     if (options.force) {
       await fs.remove(targetDir)