Load scripts in parallel instead of sequentially

To run scripts in the right order, currently we are loading them sequentially by hand (by waiting for their load event). By injecting scripts all at once, but using `defer`, we keep the execution order in place (important!), while still allowing parallelization of network requests.

Author: simonihmig

vendor/embed.js

@@ -21,6 +21,9 @@
       if (script.content) {
         scriptTag.textContent = script.content;
       }
+      scriptTag.defer = true;
+      scriptTag.async = false;
+
       head.appendChild(scriptTag);
       if (!hasSrc) {
         resolve();
@@ -29,9 +32,9 @@
   }
 
   async function injectScripts(scripts, head, host) {
-    for (let script of scripts) {
-      await injectScript(script, head, host);
-    }
+    await Promise.all(
+      scripts.map((script) => injectScript(script, head, host))
+    );
   }
 
   function injectLinks(links, head, host) {