Stub new undocumented 516 m4/m5 (#90404)

## About The Pull Request
Lummox added these and didnt document them so these are leftclicking rn

## Changelog
🆑
fix: Mouse4 and Mouse5 no longer leftclick
/🆑

Co-authored-by: TiviPlus <572233640+TiviPlus@users.noreply.com>

code/__DEFINES/_click.dm

@@ -5,6 +5,8 @@
 #define RIGHT_CLICK "right"
 #define MIDDLE_CLICK "middle"
 #define LEFT_CLICK "left"
+#define BUTTON4 "xbutton1"
+#define BUTTON5 "xbutton2"
 
 ///Mouse button that was just clicked/released
 ///if(modifiers[BUTTON] == LEFT_CLICK)

code/_onclick/click.dm

@@ -76,6 +76,9 @@
 	if(SEND_SIGNAL(src, COMSIG_MOB_CLICKON, A, modifiers) & COMSIG_MOB_CANCEL_CLICKON)
 		return
 
+	if(LAZYACCESS(modifiers, BUTTON4) || LAZYACCESS(modifiers, BUTTON5))
+		return
+
 	if(LAZYACCESS(modifiers, SHIFT_CLICK))
 		if(LAZYACCESS(modifiers, MIDDLE_CLICK))
 			ShiftMiddleClickOn(A)