Feature: Full-Duplex Wired Split (#2766)

refactor(split): Refactor split code for extension

Extract central/peripheral code to allow for plugging in alternate
transports, instead of tying all split logic to BT.

feat(split): Add full-duplex wired split support

* Depends on full-duplex hardware UART for communication.
* Supports all existing central commands/peripheral events, including
  sensors/inputs from peripherals.
* Only one wired split peripheral supported (for now)
* Relies on chosen `zmk,split-uart` referencing the UART device.

docs: Add wired split config docs.

Migrate split to its own dedicated config file, and add details
on wired split config.

Co-authored-by: Nicolas Munnich <98408764+Nick-Munnich@users.noreply.github.com>

fix: Properly override stack size on RP2040

Move the system work queue stack size override on RP2040 ouf of
a `ZMK_BLE` conditional so it is properly applied generally for that
SoC.

---------

Co-authored-by: Nicolas Munnich <98408764+Nick-Munnich@users.noreply.github.com>

docs/sidebars.js

@@ -131,6 +131,7 @@ module.exports = {
         "config/layout",
         "config/kscan",
         "config/power",
+        "config/split",
         "config/system",
         "config/studio",
       ],