Add editor features for property assignments

[spoenemann]

In cases like

MyRule returns A: ...

or

MyRule: {A} ...

where A is either a declared type (type or interface) or a rule (generating an inferred type A), we should offer editor features for the grammar language:

• Go to definition
• Document Highlight
• Document Symbols
• Find References
• More?

These require custom implementations because we don't have cross-references here, so the default services don't work.

[spoenemann]

This won't be necessary after doing #440.

[spoenemann]

Actually we still need to add the editor features mentioned above for property assignments in case the assigned property is declared in an interface.

[spoenemann]

Fixed with https://github.com/langium/langium/pull/505