add firefox extension template

externs/webextension_polyfill/storage/StorageAreaSync.hx

@@ -0,0 +1,28 @@
+package webextension_polyfill.storage;
+
+typedef StorageAreaSync = {
+	/**
+		Gets one or more items from storage.
+	**/
+	function get(?keys:ts.AnyOf3<String, Array<String>, haxe.DynamicAccess<Dynamic>>):js.lib.Promise<haxe.DynamicAccess<Dynamic>>;
+	/**
+		Gets the amount of space (in bytes) being used by one or more items.
+	**/
+	function getBytesInUse(?keys:ts.AnyOf2<String, Array<String>>):js.lib.Promise<Float>;
+	/**
+		Sets multiple items.
+	**/
+	function set(items:haxe.DynamicAccess<Dynamic>):js.lib.Promise<ts.Undefined>;
+	/**
+		Removes one or more items from storage.
+	**/
+	function remove(keys:ts.AnyOf2<String, Array<String>>):js.lib.Promise<ts.Undefined>;
+	/**
+		Removes all items from storage.
+	**/
+	function clear():js.lib.Promise<ts.Undefined>;
+	/**
+		Fired when one or more items change.
+	**/
+	var onChanged : webextension_polyfill.events.Event<(changes:StorageAreaSyncOnChangedChangesType) -> Void>;
+};