process_unicode: Make the startup overridable

Extract out the part of `qk_ucis_start` that inputs the placeholder
symbol, and make it weak, so it can be overridden.

Signed-off-by: Gergely Nagy <[email protected]>

quantum/process_keycode/process_unicode.h

@@ -37,6 +37,7 @@ struct {
 extern const qk_ucis_symbol_t ucis_symbol_table[];
 
 void qk_ucis_start(void);
+void qk_ucis_start_user(void);
 void qk_ucis_symbol_fallback (void);
 bool process_ucis (uint16_t keycode, keyrecord_t *record);