Add design instantiation hierarchy view

[nohous]

Summary

Adds a VHDL Hierarchy activity-bar view that renders the project's design instantiation tree. Picking a top entity opens a quick-pick pre-populated from the language server with real top candidates first (deepest subtree first), so the intended top of the design typically sits at the top of the list. The chosen top is persisted to vhdlls.topEntity and reloaded on the next session.

Depends on the companion language-server PR: VHDL-LS/rust_hdl#461. Until a vhdl_ls release ships with that PR, the auto-downloaded server will return method-not-found and the view will display a notice instead of failing silently.

Why

Hierarchy overview is one of the most-requested features for any HDL editor. Standard LSP doesn't have a notion of design hierarchy, so the server PR adds two custom requests; this PR consumes them.

What's added

Contributions (package.json)

• Commands: vhdlls.showHierarchy, vhdlls.setHierarchyTop, vhdlls.refreshHierarchy
• View container vhdlHierarchyContainer in the activity bar
• vhdlHierarchy view inside it, with refresh and edit-top buttons in the title bar
• Setting vhdlls.topEntity (string, e.g. work.top)

UX

• Top-entity picker. Calls vhdl/designHierarchyCandidates, splits results into "Top candidates (no incoming instantiations)" and "Other entities", sorts deepest-first within each section, and shows each entry with depth + instance count + file path. Free-text input is supported as a fallback (last item: "Type a custom entity name...").
• Tree. Each node shows instance_label : library.entity(architecture) with an icon per kind (top, bound component, unbound component, configuration, unresolved). Clicking opens the file at the instantiation site; for the top node, at the entity declaration. Notes (multi-architecture warning, cycle, unbound) appear in the tooltip.
• Persistence. Picking a top writes vhdlls.topEntity to Workspace settings (or User if no workspace is open). On next session the view auto-loads.
• Graceful degradation. If the active vhdl_ls doesn't implement the requests (LSP MethodNotFound, -32601), the view shows a notice telling the user to update or point vhdlls.languageServerUserPath at a newer build, rather than just failing.

Implementation notes

• Tree data provider uses a discriminated TreeElement union to keep status-message rows separate from real instantiation nodes (no overloaded constructor).
• Response types use a local LspLocation interface that matches the on-the-wire JSON shape (server sends uri as a string, not a deserialised Uri).
• The LanguageClient is injected through the provider; no module-level globals.

Version bumped to 0.8.0; CHANGELOG and README updated with a new "Design Hierarchy" section.

Test plan

• npm run compile clean
• Extension Development Host on a ~20-entity, 4-level-deep VHDL project: picker correctly ranks the real top first; tree renders with expected children at every level; clicking nodes navigates correctly
• Older server case: with the auto-downloaded vhdl_ls (no hierarchy support) the view shows the "method not found" notice and the picker shows the warning toast instead of erroring