Tree-sitter grammar: support match expressions

[OceanOak]

Changelog:

Tree-sitter-darklang
- Update the grammar to support match expressions

#5321

[OceanOak]

Questions:

• What should we do about wildcard eg. Error _ -> ... or | _ -> ...
  we can add support for it in the grammar, but I am not sure how it will be captured in writtenTypes. Could you please provide some insights on this?
• why don't we have MPDict?

[StachuDotNet]

• What should we do about wildcard eg. Error _ -> ... or | _ -> ...
  we can add support for it in the grammar, but I am not sure how it will be captured in writtenTypes. Could you please provide some insights on this?

It's OK to capture these as Variable match patterns.
Maybe someday we'll have a separate Wildcard MatchPattern, though -- I've thought that for a while - I'm not sure, then, what _this would be captured as (wildcard or variable?).

• why don't we have MPDict?

I'm not sure what that would look like. I don't believe similar languages have one we could use for inspiration (besides's F#'s record pattern but I don't think it perfectly applies).
I suppose the answer here is that our other match patterns are used to deconstruct structures and extract values out of them, safely, but the fields of a dict are unstable, so unsafe to deconstruct them
Also, our other patterns can be checked at parse-time, whereas a dict one (if we could imagine how it looks) probably couldn't.

[StachuDotNet]

let's add a TODO to allow for one-line match exprs (match myInt with | 1 -> ...)

[StachuDotNet]

Looking great! After adding that TODO and a test for | _ -> ..., I think this is good to go.

[OceanOak]

should we allow variable_identifierto start with _ instead of handling it this way?

[StachuDotNet]

Hmm, yeah I think so.

I've been thinking, and there's some value to (future) having a specific wildcard pattern: anything with _:

• may not be accessed
• should not be complained about, by the type-checker, for lack of usage (where we might otherwise warn "hey you captured this variable but didn't use it!")