feat(pointing): Release pressed keys on disconnect (#3204)

Match the behavior for key press release on split peripheral disconnect,
and track pressed input keys to release them as needed on disconnect.

app/include/zmk/pointing/input_split.h

@@ -7,4 +7,5 @@
 #pragma once
 
 int zmk_input_split_report_peripheral_event(uint8_t reg, uint8_t type, uint16_t code, int32_t value,
-                                            bool sync);
+                                            bool sync);
+int zmk_input_split_peripheral_disconnected(uint8_t reg);