Add easier way to set default single layer (#24376)

* Add set_single_default_layer to match set_single_persistent_default_layer
* Implements exactly the same behavior, except the write to EEPROM

quantum/quantum.h

@@ -240,6 +240,7 @@ extern layer_state_t layer_state;
 #    include "os_detection.h"
 #endif
 
+void set_single_default_layer(uint8_t default_layer);
 void set_single_persistent_default_layer(uint8_t default_layer);
 
 #define IS_LAYER_ON(layer) layer_state_is(layer)