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

---------

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

script/clippy

@@ -6,4 +6,4 @@ set -euxo pipefail
 # so specify those here, and disable the rest until Zed's workspace
 # will have more fixes & suppression for the standard lint set
 cargo clippy --release --workspace --all-features --all-targets -- -A clippy::all -D clippy::dbg_macro -D clippy::todo
-cargo clippy -p gpui
+cargo clippy -p gpui -- -D warnings