kitty: add option mouseBindings

[aionescu]

Description

This PR adds a new option programs.kitty.settings.mouseBindings which allows customizing Kitty mouse bindings in a structured way. This was previously possible via extraConfig, but the new option makes it easier.

Implementation-wise mouseBindings is very similar to the existing keybindings, just using the keyword mouse_map instead of map.

Checklist

• Change is backwards compatible.

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