[compiler] MixedConstraint parsing does not allow some array expressions

### 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:

```tsp
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 = ...;
// ~~ 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:

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

// or

alias U = "a" | "b";

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

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.