feat: add support for dir-dependency message type

[bradlc]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

The PostCSS documentation has been updated to include a new dir-dependency message type. To my understanding of webpack this should be functionally equivalent to the existing context-dependency.

Side note: would it be possible to backport this to postcss-loader v4, so that webpack 4 users can benefit?

Please feel free to make any changes to this pull request.

[codecov]

Codecov Report

Merging #524 (fca959c) into master (d9c8a7e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
+ Coverage   97.01%   97.04%   +0.03%     
==========================================
  Files           5        5              
  Lines         268      271       +3     
  Branches       87       88       +1     
==========================================
+ Hits          260      263       +3     
  Misses          8        8              

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)

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[alexander-akait]

Please accept CLA

[bradlc]

Please accept CLA

Done!

[alexander-akait]

Thanks

[bradlc]

Thanks @alexander-akait! Do you have any thoughts on backporting dir-dependency support to postcss-loader 4 for webpack 4 compatibility?

[alexander-akait]

We can do it, some developers till use tailwindcss under PostCSS v7 (just interesting)?

[bradlc]

We can do it, some developers till use tailwindcss under PostCSS v7 (just interesting)?

Yes definitely! The @tailwindcss/postcss7-compat package gets over 200,000 weekly downloads.

[alexander-akait]

Feel free to send PR to version-4 branch https://github.com/webpack-contrib/postcss-loader/branches