Delete shortcuts through the settings UI

[divyansshhh]

Problem

It would be helpful for users if they can use the settings editor to delete shortcuts too. Currently it only allows modifying but the original shortcut remains.

Additional context

Jupyterlab v4.1.2

[krassowski]

It is possible to disable default shortcuts using "disabled" attribute but it is not exposed in the settings UI.

We could either:

• (a) add "[ ] disabled" checkbox for shortcuts which would allow disabling the default shortcuts (but it would require more changes to allow adding another row for a custom shortcut)
• (b) make the shortcuts UI hide the internal fact that the default shortcuts do not get removed but disabled and make it seem in the UI as if these were getting removed (even though underneath we will be just marking the defaults as disabled) - we should then still hold the row with empty list of shortcuts to allow resetting the shortcut to defaults

I tend to think that (b) is a better option.

Cross-ref #15038