Load iframe resizer asynchronously

[akx]

Description

Please include:

• relevant motivation: when there's no internet connection, it'll take a long while for a browser to figure out that it can't load iframeResizer.contentWindow.min.js from Cloudflare.
• a summary of the change: make the script async. This ought to be safe, as there's a specific check for that. https://github.com/davidjbradshaw/iframe-resizer/blob/1ab689163f9e2505779b5f200b4f28adbddfc165/src/iframeResizer.contentWindow.js#L1292-L1302
• which issue is fixed touched upon: Gradio remains stuck in "Loading" phase in LAN configuration (not connected to the Internet) #4332

Checklist:

• I have performed a self-review of my own code
• I have added a short summary of my change to the CHANGELOG.md
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[gradio-pr-bot]

All the demos for this PR have been deployed at https://huggingface.co/spaces/gradio-pr-deploys/pr-4336-all-demos

[pngwn]

Thanks @akx!

[pngwn]

This is causing the browser tests to fail and I'm not entirely sure why. Gradio itself works fine. Will take a look later today.

[pngwn]

We've changed how we do browser tests around a bit which seems to have resolved everything here.

Sorry for the delay and thanks again @akx!