add missing externs to repo

externs/webextension_polyfill/webrequest/OnResponseStartedEvent.hx

@@ -0,0 +1,18 @@
+package webextension_polyfill.webrequest;
+
+/**
+	Fired when the first byte of the response body is received. For HTTP requests, this means that the status line and
+	response headers are available.
+**/
+typedef OnResponseStartedEvent = {
+	/**
+		Registers an event listener <em>callback</em> to an event.
+	**/
+	function addListener(callback:(details:OnResponseStartedDetailsType) -> Void, filter:RequestFilter, ?extraInfoSpec:Array<OnResponseStartedOptions>):Void;
+	/**
+		Deregisters an event listener <em>callback</em> from an event.
+	**/
+	function removeListener(callback:(details:OnResponseStartedDetailsType) -> Void):Void;
+	function hasListener(callback:(details:OnResponseStartedDetailsType) -> Void):Bool;
+	function hasListeners():Bool;
+};