[compiler] MixedConstraint parsing does not allow some array expressions

[witemple-msft]

Clear and concise description of the problem

Here are some things you cannot write in mixed constraint position that you might want to do for one reason or another:

extern dec d(target: unknown, arg: ("a" | "b")[]);
//                                            ~~ expected-token: ',' expected

// also doesn't work with a rest param
extern dec d(target: unknown, ...args: ("a" | "b")[]);
//                                                ~~ expected-token: ',' expected

// also doesn't work in other mixedconstraint contexts like `extends` clauses
alias A<T extends ("a" | "b")[]> = ...;
//                           ~~ expected-token: ',' expected

// This also doesn't work, because the checker asserts that the constraint is syntactically an array _expression_
// before evaluating the mixed constraint in rest-constraint position. But this does work in other mixed constraint
// contexts.
extern dec d(target: unknown, ...args: Array<"a" | "b">);
//                                     ~~~~~~~~~~~~~~~~ rest-parameter-array: A rest parameter must be of an array type.

This prevents writing functions/decorators that accept variadic type arguments where the valid types are those of an anonymous union.

There is a workaround. Declare an alias or a named union, then it can serve as the array item constraint:

union U {
  "a",
  "b",
}

// or

alias U = "a" | "b";

extern dec d(target: unknown, ...rest: U[]);

Checklist

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• Read the docs.
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