Find references for OML code

[0x65-e]

User Story

As a User, I would like to right-click and find all references that use my OML definition so that I can understand the context of OML definitions and quickly locate usages.

Detailed Description

Find all references is a more advanced feature of a language server. This should includes both workspace-wide references (e.g. when an explicit search for references is performed) and a passive highlight on references scoped to the current document.

Acceptance Criteria

• [x] Given a definition is highlighted, then usages of the definition in the same file should be marked
• [x] Given a definition imported by other files is highlighted, then the usages of that definition in this file and other files should show up in search

[0x65-e]

Local highlight works with the default Langium implementation after @Jeff848's fix for scope computation. Cross-file support still requires work for namespace and linking.