add missing externs to repo

externs/webextension_polyfill/runtime/MessageSender.hx

@@ -0,0 +1,35 @@
+package webextension_polyfill.runtime;
+
+/**
+	An object containing information about the script context that sent a message or request.
+**/
+typedef MessageSender = {
+	/**
+		The $(ref:tabs.Tab) which opened the connection, if any. This property will <strong>only</strong>
+		be present when the connection was opened from a tab (including content scripts), and <strong>only</strong>
+		if the receiver is an extension, not an app.
+		Optional.
+	**/
+	@:optional
+	var tab : webextension_polyfill.tabs.Tab;
+	/**
+		The $(topic:frame_ids)[frame] that opened the connection. 0 for top-level frames, positive for child frames.
+		This will only be set when <code>tab</code> is set.
+		Optional.
+	**/
+	@:optional
+	var frameId : Float;
+	/**
+		The ID of the extension or app that opened the connection, if any.
+		Optional.
+	**/
+	@:optional
+	var id : String;
+	/**
+		The URL of the page or frame that opened the connection. If the sender is in an iframe,
+		it will be iframe's URL not the URL of the page which hosts it.
+		Optional.
+	**/
+	@:optional
+	var url : String;
+};