Improve cross references for declaration merging

[msujew]

Currently, a cross reference can only ever point to a single element. For features such as declaration merging (i.e. with interfaces in TypeScript), it might make sense to adapt the cross reference resolution/grammar to allow multiple elements for a cross reference target, so that these can be merged during the scoping process.

[spoenemann]

Ideally, we would declare explicitly where we want to resolve to multiple targets in the grammar. At the declaration site, it could look like this:

interface Foo {
    myRef: @Target[]
}

But how do we specify it at the assignment? Maybe with a *?

Foo: myRef=[Target* : ID];