Fixed invalid grammar validation error

[gfontorbe]

Closes #940

Updated how assignments are collected to get rid of the following validation errors:

interface A {
	b: string
}

A  returns A: B | C;     // <-- error: Property 'b' is missing in a rule 'A', but is required in type 'A'.
AA returns A: B;         // <-- error: Property 'b' is missing in a rule 'AA', but is required in type 'A'. even if C is not referenced.

B returns A: b='foo';

interface C extends A {};
C returns A: { C } b='bar';

[msujew]

I've noticed a minor issue that we should probably address in a separate PR:

interface Y {
    event: @Event;
    state: @State;
}

Transition returns Y:
    Transition2;

Transition2: event=[Event] '=>' state=[State];

When trying to navigate to the declaration of the event assignment, nothing happens. However, when looking for references of the event property in the Y interface, we get the event assignment back. Those two functions should work in tandem. However, this is a bit unrelated to the issue at hand.

So, looks good to me 👍