Replace chrome.extension with chrome.runtime

The messaging APIs in `extension` module are deprecated in favor of
the equivalent APIs in the `runtime` module.

Sources:
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/extension
https://developer.chrome.com/extensions/extension#methods

popup.js

@@ -1,6 +1,6 @@
 document.addEventListener('DOMContentLoaded', function () {
   document.querySelector('#config').addEventListener('click', function() {
-    window.open(chrome.extension.getURL("options.html"));
+    window.open(chrome.runtime.getURL("options.html"));
   });
   
   document.querySelector('#about').addEventListener('click', function() {