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>

crates/gpui/src/platform/linux/wayland.rs

@@ -1,3 +1,6 @@
+//todo!(linux): remove this once the relevant functionality has been implemented
+#![allow(unused_variables)]
+
 pub(crate) use client::*;
 pub(crate) use client_dispatcher::*;