Add dict literal syntax

[bloodyowl]

This PR adds basic support for the dict{} literal syntax (https://github.com/rescript-lang/rescript-compiler/issues/6545).

Parsing

Semantics within the braces are similar to the record with string keys (as dict keys can be any string).

It then turns dict{"foo": "bar"} into Js.Dict.fromArray([("foo", "bar")]).

Printing

Any Js.Dict.fromArray call that contains a literal array of tuples (with no spread) is now printed as dict{...}.

I think this change is safe for the upcoming stdlib change (that'll move Js.Dict into Dict) as the change will be invisible once using this syntax. If that's something we don't want, we can add an attribute when parsing a dict{} expression so that we only print those back and leave Js.Dict.fromArray calls.