feat: add context menu and command palette support for export

[murataslan1]

Summary

This PR improves the discoverability of the "Export as Markdown" feature by adding multiple access points.

Changes

1. Editor Context Menu: Added "Export as Markdown" option that appears when right-clicking in the diagram editor panel
2. Command Palette: The export command is now accessible via the command palette (Ctrl/Cmd+Shift+P) when a diagram is active
3. Renamed Command: Changed the command title from "View Markdown Source" to "mermAId: Export Diagram as Markdown" for better clarity

Screenshots

The export option will now appear in:

• Right-click context menu in the diagram panel
• Command palette when searching for 'mermAId' or 'export'
• The existing toolbar button (unchanged)

Related Issue

Fixes #104

Testing

1. Open a diagram using @mermaid
2. Right-click in the diagram area - should see 'Export as Markdown'
3. Open command palette - search for 'mermAId export' - should see the command

[murataslan1]

Friendly follow-up — this PR adds export support via context menu and command palette. Would appreciate a review when you get a chance.