add firefox extension template

externs/webextension_polyfill/downloads/State.hx

@@ -0,0 +1,8 @@
+package webextension_polyfill.downloads;
+
+/**
+	<dl><dt>in_progress</dt><dd>The download is currently receiving data from the server.</dd><dt>interrupted</dt><dd>
+	An error broke the connection with the file host.</dd><dt>complete</dt><dd>The download completed successfully.</dd></dl>
+	These string constants will never change, however the set of States may change.
+**/
+typedef State = String;