feat: add 'Export to file' option to terminal context menu

[uadhran]

Description

This PR introduces a new "Export to file" option in the terminal's right-click context menu. This allows users to easily save the entire terminal scrollback buffer to a local text file using the system's native "Save" dialog.

Changes

• Created a new ExportContextMenu provider in tabby-terminal/src/tabContextMenu.ts.
• The provider uses the selectAll() and getSelection() methods of the terminal frontend to capture the buffer content.
• Leverages Electron's dialog.showSaveDialog to prompt the user for a file location.
• Registered the new provider in tabby-terminal/src/index.ts.
• Moved Export-to-file terminal context menu into tabby-electron (electron-only) and added frontend null guard.
• Pushed commit 2423c3a to feat/export-terminal-to-file.

Fixes

Fixes #7563

[uadhran]

Update

Export to file now appears in the terminal right‑click menu after moving it to TerminalContextMenuItemProvider.
Commit: 5a6e99d