LibJS: Implement the import assertions proposal

The hard part of parsing them in import statements and calls was already
done so this is just removing some check which threw before on
assertions. And filtering the assertions based on the result of a new
host hook.

Userland/Libraries/LibJS/Tests/modules/import-with-assertions.mjs

@@ -0,0 +1,4 @@
+import * as self from "./import-with-assertions.mjs" assert { "key": "value", key2: "value2", default: "shouldwork" };
+import "./import-with-assertions.mjs" assert { "key": "value", key2: "value2", default: "shouldwork" };
+
+export { passed } from "./module-with-default.mjs" assert { "key": "value", key2: "value2", default: "shouldwork" };