Support multi-target references

[msujew]

Adds support for multi-target references. These are references that have the possibility to target multiple elements at once. See following illustration for an example:

Note that by default, only elements that are defined on the same scope can be targeted. I.e. a multi reference as in the following will only target one element:

This behavior can be freely overwritten by adopters of Langium. However, due to the inherent complexity of aligning the References service implementation and the scoping wrt their behavior, this feature is intended for proficient Langium developers.

The intended use cases for this feature are stuff like declaration merging in TypeScript or partial classes in C#.

Contains a bunch of breaking changes:

• The whole References interface now returns arrays instead of T | undefined.
• Every service/interface/class that works with Reference in a generic manner (linker mostly) now also works with MultiReference.
• LinkingError no longer inherits ReferenceInfo (which didn't make a lot of sense in the first place), but contains a info property.

[msujew]

cc @cdietrich since you were interested in this feature.