Support default value for Reference

[ydaveluy]

It is currently possible to assign a default value to properties of type string and number.

It would be very useful to be also able to provide a default value for a reference by providing a string for the $refText attribute.

The typical use case is to provide by default a reference to an element defined in a builtin library.

E.g:
grammar:

entry Model:
    elements+=(Primitve|Type) *;

interface Primitive {
    name:string
}
interface Type {
    name:string
    primitive:@Primitive = 'int32' /* default reference text */
}
Primitve returns Primitive:
    'primitive' name=ID;

Type returns Type:
    'type' name=ID ('extends' primitive=[Primitive:ID])?;

hidden terminal WS: /\s+/;
terminal ID: /[_a-zA-Z][\w_]*/;

builtin library:

primitive int8
primitive int16
primitive int32
primitive int64

model:

type T1 /* extends int32 by default */
type T2 extends int8

I can provide a PR for this request.

[msujew]

I generally like the idea, but I have two misgivings:

1. Where do we represent a linking error on the missing reference? (i.e. how can the user be informed that the reference resolving failed; We don't have a corresponding CST node for the reference)
2. I don't really like the way this dilutes the type system of the grammar language (as it would kind of allow to assign strings to references). This is just a minor issue.

I feel like (1) in particular is rather problematic in the general case. I think this should be rather handled in the adopter code as something like primitive.ref ?? int32Primitive, since it is an adopter concern that the variable is available for referencing in the first place.