Extensions: Restore declared script dependencies

[sirreal]

With a part of our build tooling supplying WordPress script dependencies (#11591), we removed the ability to declare script dependencies explicitly.

There are cases that are not yet covered by the tooling —https://github.com/Automattic/jetpack/pull/9802/files#r272845657. Restore the ability to declare script dependencies.

Testing instructions:

• No regressions, scripts continue to include automatic dependencies, polyfill is always included.
• Membership Block #9802 will be able to declare thickbox dependency.

[jetpackbot]

	Warnings
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

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Generated by 🚫 dangerJS against 9a3e690

[simison]

Looks good and tested that editor and view side still work. 👍

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello sirreal! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D26652-code before merging this PR. Thank you!

[kraftbj]

@sirreal When you merge this, could you handle the wp.com merge? D26652-code now contains a few different PRs worth of class-jetpack-gutenberg.php changes (including D26525-code from #11591).

Marking the wpcom test as passed since it fails as this partially reverts an uncommitted WP.com side change.

[sirreal]

I've created D26666-code to handle the full sync. This includes a change to the way dependencies are read, so I'll need to include an extensions build as well.