Add option to toggle keyboard shortcuts on/off

[Sukhendu2002]

What?

Closes: #69146

This PR adds a toggle option in the keyboard shortcuts help modal that allows users to enable or disable all keyboard shortcuts globally throughout the editor.

Testing Instructions

1. Open the WordPress editor and create or edit a post
2. Open the keyboard shortcuts modal
3. Observe the new toggle control at the top of the modal labeled "Enable keyboard shortcuts"
4. Toggle it off and close the modal

Screenshots or screencast

[Mamaduka]

Thanks for contributing, @Sukhendu2002.

I think this needs to be discussed first before we move forward with implementation.

[t-hamano]

Perhaps users would prefer to toggle shortcuts individually, rather than all at once. Additionally, a more ideal approach would be to make shortcuts customizable. See #3218

[Mamaduka]

Right, configurable shortcuts seem more flexible solutions.