feat(stylus): update Stylus grammar

[fvsch]

This PR updates the Stylus grammar.

The main motivation for this PR was to convert Stylus.tmLanguage (PLIST) to JSON, for reasons discussed in #5:

• We've had parsing errors with the XML PLIST format for Vue, and maybe that wasn't about XML-vs-JSON at all, but I’d feel safer with only using JSON since 95%+ of the grammars we use are JSON. Fewer things that can go wrong.
• In our Enterprise product, we've had some issues where requests for *.tmLanguage URLs would not get appropriate HTTP headers, which was not an issue with *.tmLanguage.json. This should be fixed by now, but again: using common Web formats and file extensions leads to fewer things that can go wrong.

For the source of this updated Stylus grammar, I found two possible sources:

• The main Stylus repository has an 8 year old PLIST, which is probably what we currently use: https://github.com/stylus/stylus/blob/dev/editors/Stylus.tmbundle/Syntaxes/Stylus.tmLanguage
• The main Stylus VSCode extension uses a more recent JSON grammar: https://github.com/d4rkr00t/language-stylus/blob/master/syntaxes/stylus.json

So here I used the second one. If it's good enough for the main Stylus VSCode extension, it should work for us.

[SamVerschueren]

Just a question on the change in the grammar file. Other than that this looks good. Thanks for looking into this 🙏 !

[SamVerschueren]

Is there a base stylus grammar on Monaco? Can't recall exactly but I think that's what this is meant for?

[fvsch]

This configuration object resolves to a path, "stylus/language-configuration.json". If it's not specified, we infer a default path from the language, as ${language}/language-configuration.json.

I’ve taken to make the configuration and grammar paths more explicit instead of convention-based, mostly because I like when stuff is explicit ^^