fix(grit): fix matching multiple args

[arendjr]

Summary

We had an issue with matching function arguments in Grit queries, that prevented metavariables from matching multiple arguments. In short, the metavariable was consumed by the JsFormalParameter and stored inside one of its slots. This wasn't a problem if you only tried to match a single argument with no other syntax, but if you wanted the metavariable to match anything more, it wouldn't work.

To fix it, I made sure JsMetavariable can be accepted in our grammar next to JsFormalParameter and updated the JS parser so it's accepted there.

I also extended our list handling in the bindings a bit to be more in line with how Grit's reference implementation does it. The change in GritBinding::singleton() seems it's actually necessary, but some of the others may not be. It's probably better this way, but we don't have the test coverage yet to say for sure 😅

Test Plan

Added a test case.

[codspeed-hq]

CodSpeed Performance Report

Merging #3797 will degrade performances by 9.14%

_{Comparing arendjr:fix-matching-multiple-args (adfb11a) with main (3f1790e)}

Summary

❌ 1 regressions
✅ 106 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	arendjr:fix-matching-multiple-args	Change
❌	db_17847247775464589309.json[cached]	12.9 ms	14.2 ms	-9.14%