Add support for running initialization scripts before content

This adds the ability to run  a script in a separate global before content gets run. This code has access to additional abilities, notably including the `defineBuiltinModule` function.

Building on this will allow us to change ComponentizeJS to support content using imports from `wasi:package-name` module names without rewriting the code first. That in turn will enable support for non-bundled content code.

Author: tschneidereit

tests/e2e/init-script/expect_serve_stdout.txt

@@ -0,0 +1 @@
+stdout [0] :: Log: foo

tests/e2e/init-script/init-script.js

@@ -0,0 +1,8 @@
+import { func } from "builtinMod";
+async function handle(event) {
+  console.log(func());
+  return new Response(func());
+}
+
+//@ts-ignore
+addEventListener('fetch', (event) => { event.respondWith(handle(event)) });

tests/e2e/init-script/init.js

@@ -0,0 +1,8 @@
+const builtinMod = {
+    func() {
+        return 'foo';
+    }
+}
+
+defineBuiltinModule('builtinMod', builtinMod);
+print("initialization done");