Linux/x11 input handling (#7811)

Implements the basics of keyboard and mouse handling.
Some keys will need special treatment, like Backspace/Delete. In this
PR, all keys are treated as append-only. Leaving this for a follow-up.

I used @gabydd 's branch as a reference (thank you!) as well as
https://github.com/xkbcommon/libxkbcommon/blob/master/doc/quick-guide.md
For future work, I'll also use
https://github.com/xkbcommon/libxkbcommon/blob/master/tools/interactive-x11.c

All commits are separately compileable and reviewable.

Release Notes:
- N/A

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Co-authored-by: Mikayla Maki <mikayla@zed.dev>