trsync: Introduce new "trigger synchronization" support

Separate out the stepper stopping code from endstop.c into its own
trsync.c code file.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stepper.h

@@ -4,7 +4,5 @@
 #include <stdint.h> // uint8_t
 
 uint_fast8_t stepper_event(struct timer *t);
-struct stepper *stepper_oid_lookup(uint8_t oid);
-void stepper_stop(struct stepper *s);
 
 #endif // stepper.h