[202_106] display the shortcut keys corresponding to the symbols in the right footer

[Prthmsh7]

PR for #2886

Before :

After :

Root cause: The footer had no mechanism to look up or display keyboard shortcuts
Fix: Added get_shortcut_suffix() which calls Scheme's kbd-find-inv-binding to resolve the shortcut for symbols, and used the existing kbd_shortcut() for operations which appends the result as [Shortcut] in the right footer.

How to Test

1. Open a document in the editor.
2. Move the cursor to a position where a symbol is present.
3. Verify that the right footer displays the corresponding keyboard shortcut.
4. Move the cursor away from the symbol and confirm that the footer clears.