Assets: Fix defer for async scripts

[tyxla]

I've noticed that whenever we register a script as async, it will add a defer attribute to the translations script tag that is rendered before the enqueued script tag.

This PR fixes that behavior so we defer the script we enqueued instead.

I'm also adding a couple of tests to confirm that the behavior is working as expected.

Changes proposed in this Pull Request:

Assets: Defer the enqueued script instead of its translations

Jetpack product discussion

No related product discussion

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Verify package tests pass.
• Verify that if you add async => true to a script you register, defer will be added to the script and not to its translations.

[github-actions]

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[tyxla]

Thanks for the feedback @anomiex! I've addressed it and added several comments to suppress false positive phpcs errors.

Ready for another look.

[anomiex]

Let's go for it