Add the ability to use parentheses when declaring `TypeAlternatives`

The Langium grammar allows to create the rule X, where the property x is an array of type alternatives, and it has to be possible to declare the corresponding type XType, as in the example. We should support parentheses in TypeAlternatives.

X returns XType:
    x+=(ID | NUMBER)*;

interface XType {
    x: (number | string)[];
}

[pluralia]

I would personally prefer an array of strings over mixed strings and numbers, so yes I think the restriction should be ok.

The ability to have an array of strings will remain after the parentheses are added, also I think it’s a “low hanging fruit”, so I’d suggest to implement it.

[spoenemann]

Would it be okay?

I would personally prefer an array of strings over mixed strings and numbers, so yes I think the restriction should be ok. This is related to https://github.com/langium/langium/issues/494#issuecomment-1125811597, where I proposed to disallow multiple assignments with different types to the same property. But maybe I’m too strict here and there could be valid use cases to write explicit alternatives to an assignment.

An important point I should mention: property types in the current implementation are able to support brackets, and it won’t be that “expensive” to implement.

If it’s a “low hanging fruit”, we could still implement this, independently of how we decide about validation.