add firefox extension template

externs/webextension_polyfill/urlbar/SearchOptions.hx

@@ -0,0 +1,13 @@
+package webextension_polyfill.urlbar;
+
+/**
+	Options to the <code>search</code> function.
+**/
+typedef SearchOptions = {
+	/**
+		Whether to focus the input field and select its contents.
+		Optional.
+	**/
+	@:optional
+	var focus : Bool;
+};