Support DocumentSymbolProvider in CustomTextEditor

Fixes #97095

Description

This PR adds a proposed extension API that allows Custom Editor extensions to provide outline data for the Outline view, Breadcrumbs, and Go to Symbol quick-pick. Previously, custom editors had no way to contribute to these features since they don't use a standard text model that a DocumentSymbolProvider can work with.

New API

The proposed API customEditorOutline adds:

• CustomEditorOutlineItem — describes a node in the outline tree with id, label, detail, tooltip, icon, contextValue, and children.
• CustomEditorOutlineProvider — interface with provideOutline(), revealItem(), onDidChangeOutline, and onDidChangeActiveItem.
• window.registerCustomEditorOutlineProvider(viewType, provider) — registers the provider for a specific custom editor view type.

Features

• Outline tree: Extension provides items via provideOutline() → they appear in the Outline view with icons, labels, and a tree hierarchy.
• Active element tracking: Extension fires onDidChangeActiveItem → the outline highlights/follows the active node.
• Reveal on click: User clicks an outline node → revealItem(id) is called so the extension can scroll the webview.
• Breadcrumbs: Automatically built from the active item's parent chain.
• Go to Symbol: Items appear in the Ctrl+Shift+O quick-pick.
• Inline toolbar buttons: Extensions contribute to customEditor/outline/toolbar with "group": "inline" in their package.json.
• Overflow menu: Extensions contribute to customEditor/outline/toolbar without the inline group.
• Right-click context menu: Extensions contribute to customEditor/outline/context (separate from the toolbar menu).
• When clauses: The customEditorOutlineItem context key is set to each item's contextValue, enabling conditional menu contributions.

Two Separate Menu Contribution Points

Architecture

Extension Host                    Main Thread
─────────────────                 ──────────────────────────────
ExtHostCustomEditorOutline  ←RPC→  MainThreadCustomEditorOutline
                                          │
                                          ▼
                                  ICustomEditorOutlineProviderService
                                          │
                                          ▼
                                  CustomEditorExtensionOutline (IOutline<>)
                                          │
                                          ▼
                                  OutlinePane / Breadcrumbs / QuickPick

Files Changed

New files

Modified files

How to Test

1. Use an extension that registers a custom editor (e.g. a visual designer with CustomTextEditorProvider).
2. Add "enabledApiProposals": ["customEditorOutline"] to the extension's package.json.
3. Call vscode.window.registerCustomEditorOutlineProvider(viewType, provider) with an implementation that returns outline items.
4. Open a file handled by the custom editor.
5. Verify the Outline view shows the items from the provider with correct icons, labels, and hierarchy.
6. Verify clicking an outline node calls revealItem().
7. Verify firing onDidChangeActiveItem highlights the node in the outline.
8. Verify Breadcrumbs shows the active item's parent chain.
9. Verify Ctrl+Shift+O shows the items in the quick-pick.
10. If the extension contributes to customEditor/outline/toolbar, verify inline buttons appear on hover and respond to when clauses using customEditorOutlineItem.
11. If the extension contributes to customEditor/outline/context, verify the right-click context menu appears with the correct items filtered by when clauses.