Add partial YouTube compatibility shim

ext/src/compat/youtube.js

@@ -0,0 +1,10 @@
+"use strict";
+
+// Insert script before first script is run
+document.addEventListener("beforescriptexecute", function onBeforeScriptExecute () {
+    document.removeEventListener("beforescriptexecute", onBeforeScriptExecute);
+
+    const scriptElement = document.createElement("script");
+    scriptElement.src = browser.runtime.getURL("vendor/webcomponents-lite.min.js");
+    document.head.prepend(scriptElement);
+});