add firefox extension template

externs/webextension_polyfill/userscripts/Static.hx

@@ -0,0 +1,9 @@
+package webextension_polyfill.userscripts;
+
+typedef Static = {
+	/**
+		Register a user script programmatically given its $(ref:userScripts.UserScriptOptions),
+		and resolves to a $(ref:userScripts.RegisteredUserScript) instance
+	**/
+	function register(userScriptOptions:UserScriptOptions):Void;
+};