Emit metadata with union constituents

[tinganho]

Right now, when we emit meta data of one property that is defined as:

class A {
    @Property()
    p1: 's1' | 's2';
}

We emit the metadata as:

__metadata("design:type", String)

So, whoever is using this metadata cannot know the literal types of p1. A library author has to decorate the literal types s1 and s2 in order to access these literal types:

class A {
    @Property(strings: ['s1', 's2'])
    p1: 's1' | 's2';
}

Why don't we just emit it as an array instead?

__metadata("design:type", ['union', 's1', 's2'])

Related to:
#12703

[mhegazy]

The metadata provided are not meant to be a reflective runtime type system. you can find more discussion about this in #3060 and #3628

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.