Updates send_string functionality, adds terminal feature (#1657)

* implement basic terminal stuff

* modify send_string to read normal strings too

* add files bc yeah. working pgm detected

* pgm detection apparently not working

* adds send string keycodes, additional keycode support in send string

* implement arguments

* [terminal] add help command

* [terminal] adds keycode and keymap functions

* [terminal] adds nop.h, documentation

* update macro docs

quantum/quantum_keycodes.h

@@ -431,6 +431,11 @@ enum quantum_keycodes {
     KC_LOCK,
 #endif
 
+#ifdef TERMINAL_ENABLE
+    TERM_ON,
+    TERM_OFF,
+#endif
+
     // always leave at the end
     SAFE_RANGE
 };