Extract sendstring into its own compilation unit (#12060)

* Extract sendstring into its own compilation unit

* License headers?

* Put this include in the header

common_features.mk

@@ -17,6 +17,7 @@ SERIAL_PATH := $(QUANTUM_PATH)/serial_link
 
 QUANTUM_SRC += \
     $(QUANTUM_DIR)/quantum.c \
+    $(QUANTUM_DIR)/send_string.c \
     $(QUANTUM_DIR)/bitwise.c \
     $(QUANTUM_DIR)/led.c \
     $(QUANTUM_DIR)/keymap_common.c \