add missing externs to repo

externs/webextension_polyfill/windows/WindowState.hx

@@ -0,0 +1,7 @@
+package webextension_polyfill.windows;
+
+/**
+	The state of this browser window. Under some circumstances a Window may not be assigned state property,
+	for example when querying closed windows from the $(ref:sessions) API.
+**/
+typedef WindowState = String;