add missing externs to repo

externs/webextension_polyfill/namespaces/devtools_panels/devtoolspanels/ExtensionSidebarPane.hx

@@ -0,0 +1,28 @@
+package webextension_polyfill.namespaces.devtools_panels.devtoolspanels;
+
+/**
+	A sidebar created by the extension.
+**/
+typedef ExtensionSidebarPane = {
+	/**
+		Sets an expression that is evaluated within the inspected page. The result is displayed in the sidebar pane.
+	**/
+	function setExpression(expression:String, ?rootTitle:String):js.lib.Promise<ts.Undefined>;
+	/**
+		Sets a JSON-compliant object to be displayed in the sidebar pane.
+	**/
+	function setObject(jsonObject:ts.AnyOf3<String, Array<Any>, haxe.DynamicAccess<Any>>, ?rootTitle:String):js.lib.Promise<ts.Undefined>;
+	/**
+		Sets an HTML page to be displayed in the sidebar pane.
+	**/
+	function setPage(path:String):Void;
+	/**
+		Fired when the sidebar pane becomes visible as a result of user switching to the panel that hosts it.
+	**/
+	var onShown : webextension_polyfill.events.Event<(window:js.html.Window) -> Void>;
+	/**
+		Fired when the sidebar pane becomes hidden as a result of the user switching away from the panel that hosts the sidebar
+		pane.
+	**/
+	var onHidden : webextension_polyfill.events.Event<() -> Void>;
+};