Add the function of changing keyboard shortcuts

Hello developer, the windterm software you developed is very easy to use. I often use it to connect to my server, but I am not used to the default keyboard shortcut settings. It would be great if I could customize the shortcut keys.

The issue names no files, tests, or entry points. Start by locating how WindTerm handles keyboard shortcuts and clarify which shortcuts and configuration flow are in scope; done should mean users can customize the agreed shortcuts and use the resulting bindings reliably.

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