add missing externs to repo

externs/webextension_polyfill/sessions/Session.hx

@@ -0,0 +1,20 @@
+package webextension_polyfill.sessions;
+
+typedef Session = {
+	/**
+		The time when the window or tab was closed or modified, represented in milliseconds since the epoch.
+	**/
+	var lastModified : Float;
+	/**
+		The $(ref:tabs.Tab), if this entry describes a tab. Either this or $(ref:sessions.Session.window) will be set.
+		Optional.
+	**/
+	@:optional
+	var tab : webextension_polyfill.tabs.Tab;
+	/**
+		The $(ref:windows.Window), if this entry describes a window. Either this or $(ref:sessions.Session.tab) will be set.
+		Optional.
+	**/
+	@:optional
+	var window : webextension_polyfill.windows.Window;
+};