feat: Add ability to 'negate' keybindings.

[lucyleeow]

Description

I am not 100% sure that this functionality belongs in app-model, but I just wanted to get your thoughts @tlambert03

Negating a keybindings is a functionality in VScode and replicated in napari/napari#6204 . It is a way to make a specific keybinding inactivate, done so by adding a '-' to the start of a command ID string.

The reason is because we do not want to delete 'default' keybindings (i.e. app and plugin keybindings), thus we would like a way to make these inactive without removing them from the keybinding registry. This allows us to:

• 'restore default' keybindings
• when a use 'overrides' a default keybinding with a new one (with a different key sequence) we need to ensure the over-ridden default keybinding is inactive (i.e., pressing the default keysequence does not do anything)

If we have functionality to store different sources of keybindings (#226), it seems reasonable (??) that this functionality (or something similar) would be useful. But I would understand if you think that this is too specific/up to individual users to implement and tailor for their own use.

Implementation:

We thought that it could be implemented in a method in keybinding registry or at dispatch. Implementing it as a registry method would allow it to be used for purposes outside of dispatch (maybe for e.g., command palette purposes?)

cc @DragaDoncila @dalthviz