Block Support Styles: Add priority param

[ockham]

Backport of WordPress/gutenberg#41015.

Needed for the package-sync "dry run" PR: #3154 (comment)

Questions:

• Should this include other related changes, e.g. Global styles: specify priority in remove_action gutenberg#43073?
• Does this need a (basic) unit test? Edit: I'm leaning towards No, see Block Support Styles: Add priority param #3158 (comment).

cc/ @glendaviesnz @ramonjd for review 😊

Gutenberg tracking issue: WordPress/gutenberg#43440
Trac ticket: https://core.trac.wordpress.org/ticket/56467

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[ramonjd]

Thanks @ockham LGTU(s)

[ockham]

I've filed a counterpart PR for GB: WordPress/gutenberg#43779

[ockham]

I've thought about adding a unit test, but I'm now leaning against, for the following reasons:

• wp_enqueue_block_support_styles is pretty straight-forward (especially compared to other functions in the same file -- most of which don’t have unit test coverage.) -- It basically just calls add_action().
• It passes a closure to add_action(). This means we can't really have a test that checks for that closure (e.g. via has_action(), as we can't reference it from outside the function scope.
• AFAICS, this leaves us with some indirect testing, such as running wp_enqueue_block_support_styles(), and then wp_head() (or wp_footer(), respectively), to see if the CSS that was passed to the function was added to the header (or footer, respectively). However, it's kinda messy to look for that styling inside of the (fairly big) header/footer.
• Finally, there isn't really any way to test if the $priority was respected -- which is the change we're actually making here.

[desrosj]

Merged in https://core.trac.wordpress.org/changeset/54048.