Enable Render on Save toggle in visual editor

[juliasilge]

The Render on Save checkbox appears in the visual editor's action bar, but it wasn't correctly working there. This PR makes the checkbox (more) correctly functional in the visual editor. I'll be honest that there is still some weirdness with the quarto.render.renderOnSave setting, but this is an improvement.

This is the Quarto-side follow-up called out in posit-dev/positron#16022, which is making extension-contributed action bar checkboxes follow their command's enablement expression. cc @dhruvisompura

Root cause

Two issues:

1. The quarto.toggleRenderOnSave command's enablement was editorLangId == quarto, which doesn't match when the visual editor (a custom editor) is active so the checkbox was not fully/correctly enabled there.
2. The quarto.editor.type / quarto.editor.renderOnSave / quarto.editor.renderOnSaveShiny context keys that drive the checkbox's checked state were only updated via onDidChangeActiveTextEditor, which custom editors don't fire, so the state was stale even where the checkbox was visible.

Changes

• package.json: add activeCustomEditorId == 'quarto.visualEditor' to the command's enablement, matching the pattern used by other menu contributions.
• editor.ts: add an onDidChangeActiveEditor event to the visual editor tracker, fired from each tracked webview panel's onDidChangeViewState when its panel becomes active, and expose it via VisualEditorProvider.onDidChangeActiveEditor().
• context-keys.ts: refactor setEditorContextKeys / setLanguageContextKeys to take a TextDocument (they only ever used the document), subscribe to the new event so context keys refresh when a visual editor gains focus, and fall back to the active visual editor's document in the configuration-change listener.

Validation

• Open a .qmd in visual mode: the checkbox appears and is clickable.
• Toggle the checkbox in visual mode and save: render kicks off when checked, doesn't when unchecked.
• Toggle in visual mode, switch to source mode (and back): state carries across.
• Regression: source-mode behavior unchanged; checkbox stays hidden for non-Quarto files.

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