Improve syntax highlighting

[jiribenes]

Changes

• parse comments precisely
• support splices in (currently all) strings
• support individual characters
• support escapes in strings and characters
• support multi-line strings
• support namespace declarations
• parse capability sets precisely
• add folding markers
• support float literals
• parse typed holes in line with the rest of the file
• add operator support (only visible in some themes)
• in is now a conditional keyword (rendered as a keyword only in var x in r = ...)
• fix definitions being stuck at => and then not parsing the rest of the arguments
• highlight at correctly
• highlight invalid escapes
• highlight modules and imports correctly
• add support for region

Caveats / open questions

• currently, we only highlight '\uA0EB' char literals as opposed to \uA0EB detected in the parser, see issue Escapes are broken effekt#521

Future work

• precise parsing of extern defs
• add support for identifiers that use ? and ! (I tried, but I couldn't get it to work)
• add a test suite (low-prio, but it would be nice to know we're somewhat stable)
• use semantic highlighting from the compiler via LSP: https://code.visualstudio.com/api/language-extensions/semantic-highlight-guide, then we could even distinguish an effect type from a normal type, a parameter from a variable, etc. :)
  • demo gifs here: Proposal of the semantic highlighting protocol extension microsoft/vscode-languageserver-node#367 (comment)
• support box {<capabilities>*} { ... } syntax

Screenshots (before [left] vs after [right])

[jiribenes]

I hereby reject the way character escapes are actually parsed, so the grammar is exactly what you would expect (moreover, we're somewhat lax in comparison with the actual lexer in order to allow more characters in a char literal, which is -- I think -- a good thing for highlighting)

[jiribenes]

I was quite surprised, but \uFF0F is really a char literal with no need for single quotes in the current parser!

[jiribenes]

Filed effekt-lang/effekt#521 just to be sure that we don't have to support this :)

[jiribenes]

I'll keep better support for externs for later, but here's a sneak peek:

[jiribenes]

I've updated the screenshots with some of the newer features, but I'm running out of things to add, so feel free to review and merge :)