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>

app/Kconfig.defaults

@@ -1,6 +1,10 @@
 # Copyright (c) 2024 The ZMK Contributors
 # SPDX-License-Identifier: MIT
 
+config SYSTEM_WORKQUEUE_STACK_SIZE
+    default 2048 if SOC_RP2040
+    default 2048 if ZMK_BLE
+
 # HID
 if ZMK_HID_REPORT_TYPE_HKRO