Add support for asserts in type declarations

[guillaumebrunerie]

Currently, the following (valid) code doesn’t parse:

declare const f: (x: any) => asserts x;
declare const g: (x: any) => asserts x is number;

It does not parse because the grammar only supports the asserts keyword when it follows a colon (as in function f(x: any): asserts x {}). This pull request adds support for asserts in type declarations as above.

I removed the colon from the $.asserts rule and added a new rule $.asserts_annotation which adds the colon back (so that we can use $.asserts in places that do not have a colon). This mirrors the way the existing $.type_annotation and $.type_predicate_annotation work, but unfortunately it modifies the parse tree of existing code. But I think this change still makes sense.

Checklist:

• [X] All tests pass in CI.
• [X] There are sufficient tests for the new fix/feature.
• [x] Grammar rules have not been renamed unless absolutely necessary.
• [X] The conflicts section hasn't grown too much.
• [X] The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c). (STATE_COUNT increased from 4437 to 4451)

[guillaumebrunerie]

Closing in favor of #252.