Implement `Loader` (but not yet use)

[mizdra]

ref: #45 #33

This is an alternate implementation of css-modules-loader-core (a library that returns tokens exported by CSS Modules files). There are some important differences to the original implementation.

• It returns not only the token name, but also the location of the token on the original CSS Modules file.
  • This allows the user to know where the token came from in the file.
  • ref: https://github.com/mizdra/checkable-css-modules/blob/0fd040fc6d1fd30a74c015394f62e8fcae9053bf/test/loader.spec.ts#L18-L68
• It uses postcss-modules to collect the tokens exported by the CSS Modules file.
  • The css-modules-loader-core implemented its own collecting logic.
  • Since the better collecting logic was already implemented in postcss-modules, I used it in the alternate implementation.
• It supports CSS Modules files written in AltCSS.
  • It can be combined with the sass compiler and the less compiler using the transform option
  • The token location information returned by the Loader is mapped onto .scss or .less (using source mapping).
  • ref: https://github.com/mizdra/checkable-css-modules/blob/0fd040fc6d1fd30a74c015394f62e8fcae9053bf/test/loader.spec.ts#L556-L697
• It handles tokens in two different types: "local tokens" and "external tokens".
  • External tokens are tokens imported from external files using @import or composes property.
  • Local tokens are tokens that are not external tokens.
  • This allows the user to know what file the token came from.
  • ref: https://github.com/mizdra/checkable-css-modules/blob/0fd040fc6d1fd30a74c015394f62e8fcae9053bf/test/loader.spec.ts#L70-L324
• It supports postcss@8
  • css-modules-loader-core does not support postcss@8.
• Its interface is not compatible with css-modules-loader-core.
  • I learn from the past and start over in a better way. 🚀🚀🚀

[mizdra]

Completed!

[mizdra]

The classification of local and external tokens could be improved a bit.