Add usage-count CodeLens for variables and functions

[julia-script]

Summary

Add LSP CodeLens usage counts for local variables, parameters, functions, and other suitable declarations.

This builds on the workspace reference product in #179. Local declarations can use current per-module semantic occurrences; top-level declarations need complete source-root reference summaries.

Scope

• Advertise and implement textDocument/codeLens.
• Use canonical semantic identities rather than textual name counts.
• Count local binding and parameter references from current module/function facts.
• Count top-level function/declaration references across closed reverse-dependent modules through Index workspace references beyond the open-root import closure #179.
• Support codeLens/resolve so expensive workspace counts can be deferred.
• Keep lenses coherent with the accepted document and workspace revision.
• Define and document:
  • whether import-clause occurrences count as usages;
  • whether declarations are excluded from counts;
  • whether zero-count lenses are hidden or displayed;
  • whether local and workspace counts are presented differently.

Acceptance criteria

• A local variable and parameter lens reports the exact semantic reference count within its scope.
• A top-level function lens includes uses from unopened source-root modules.
• Shadowed or same-spelled declarations do not affect each other's counts.
• The declaration occurrence itself is not accidentally counted as a reference.
• Stale lens resolution does not combine current text with old reference summaries.
• Lens ordering and titles are deterministic.
• Large files and workspaces do not eagerly recompute every global count for every request.
• Tests cover imports/aliases, first-class callable uses, direct calls, zero usages, unsaved buffers, and closed reverse dependents.

Dependency

Blocked by the workspace reference completeness and freshness semantics in #179.