Extensible split data sync (#11930)

* Extensible split data sync capability through transactions.

- Split common transport has been split up between the transport layer
  and data layer.
- Split "transactions" model used, with convergence between I2C and
  serial data definitions.
- Slave matrix "generation count" is used to determine if the full slave
  matrix needs to be retrieved.
- Encoders get the same "generation count" treatment.
- All other blocks of data are synchronised when a change is detected.
- All transmissions have a globally-configurable deadline before a
  transmission is forced (`FORCED_SYNC_THROTTLE_MS`, default 100ms).
- Added atomicity for all core-synced data, preventing partial updates
- Added retries to AVR i2c_master's i2c_start, to minimise the number of
  failed transactions when interrupts are disabled on the slave due to
  atomicity checks.
- Some keyboards have had slight modifications made in order to ensure
  that they still build due to firmware size restrictions.

* Fixup LED_MATRIX compile.

* Parameterise ERROR_DISCONNECT_COUNT.

common_features.mk

@@ -537,7 +537,11 @@ ifeq ($(strip $(SPLIT_KEYBOARD)), yes)
 
     # Determine which (if any) transport files are required
     ifneq ($(strip $(SPLIT_TRANSPORT)), custom)
-        QUANTUM_LIB_SRC += $(QUANTUM_DIR)/split_common/transport.c
+        QUANTUM_SRC += $(QUANTUM_DIR)/split_common/transport.c \
+                       $(QUANTUM_DIR)/split_common/transactions.c
+
+        OPT_DEFS += -DSPLIT_COMMON_TRANSACTIONS
+
         # Functions added via QUANTUM_LIB_SRC are only included in the final binary if they're called.
         # Unused functions are pruned away, which is why we can add multiple drivers here without bloat.
         ifeq ($(PLATFORM),AVR)