Add linter for Less

[cl8n]

mostly so things can be fixed automatically and you don't have to care about formatting so much. the enabled rules that can't be fixed automatically seem like good suggestions and already had very few errors in this project

• will conflict with #10568 because postcss dependency
• I haven't tested the formatting for GitHub action
• some of the rules should probably be warnings instead of errors

[nanaya]

I don't think stylelint-stylistic is supposed to be used for new projects.

[cl8n]

I looked into stylelint's recommended migration path for their stylistic rules (tl;dr try Prettier) -- it doesn't really align with the goal I had here of being non-intrusive. I wanted the result to be similar to osu-web's ESLint config in that you can still make some formatting decisions yourself where it makes sense, it just lints the tedious things that we agree should always be done a certain way

so I went with their next recommendation which is to depend on someone else maintaining similar rules, and this is the most popular option.

[nanaya]

prettier can be configured to only handle css files

I took a quick look of its output and it seems fine (apart of mod names need quoting)

as for customization, I think we would care more about customizing actual lint than formatting

[nanaya]

The formatter part is now done using prettier (#11355). The linting part would need some more considerations. Closing this one for now and feel free to make another pr if you want to add stylelint.