Enforce sync JS

[timkelty]

Summary

• Load appended scripts sequentially via native script insertion.
• Remove the unused app/resource-js fallback.

Why

CP resources may be served from a CDN on another domain. Without this change, appended dependent scripts can execute out of order for cross-domain requests.

Example:

<script src="https://cdn.craft.cloud/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx/build/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx/cpresources/vendor.js"></script>
<script>window.App.init();</script>

window.App.init() can run before vendor.js finishes loading.

Previous fix

The earlier app/resource-js fallback was introduced in 1a958a7. It could appear to help because it converted cross-domain CP resource URLs into same-origin requests, keeping jQuery on its same-origin script-loading path. But the append flow still relied on jQuery's script handling, so execution order for dependent appended scripts was not guaranteed.

Relevant jQuery issues:

• append() doesn't guarantee sequential execution of <script> tags jquery/jquery#4801
• DOM inserted scripts should run asynchronously jquery/jquery#1895

This PR preserves ordering explicitly and removes that fallback path.

Related issues:

• #13751
• #13752