Various fixes for valid code not being parsed properly

[guillaumebrunerie]

This pull request fixes five instances of valid code not being parsed correctly (mostly due to new or somewhat advanced features):

• Support for the unique symbol type (see here):

const sym: unique symbol = Symbol();

• Correct parsing of as const (it is currently being parsed as if const were a type, but it is a keyword):

const str = "foobar" as const;

• Support for asserts in type declarations (see also longer description at Add support for asserts in type declarations #250):

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

• Support for const type parameters (see here):

const f = <const T extends string>(x: T) => x;

• Support for extends clauses in infer (see here):

type StringAsNumber<T> = T extends `${infer N extends number}` ? N : never;

Checklist:

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

[amaanq]

Thanks for your work!