Add textual/visual mode toggle buttons for Neva editors

[emil14]

Summary

• add editor title toggle commands for Textual/Visual modes in .neva files
• track active Neva editor + current mode via VS Code context keys
• expose a minimal extension API surface (getEditorMode, setEditorMode, onDidChangeEditorMode) as a starting point for visual editor integration

Verification

• npm run build
• open a .neva file in Extension Development Host
• verify editor title shows Visual button by default
• click Visual and confirm placeholder message + button flips to Textual
• click Textual and confirm button flips back to Visual

Closes #26