add missing externs to repo

externs/webextension_polyfill/sessions/Device.hx

@@ -0,0 +1,13 @@
+package webextension_polyfill.sessions;
+
+typedef Device = {
+	var info : String;
+	/**
+		The name of the foreign device.
+	**/
+	var deviceName : String;
+	/**
+		A list of open window sessions for the foreign device, sorted from most recently to least recently modified session.
+	**/
+	var sessions : Array<Session>;
+};