Enhance code quality related to WP_Theme_JSON::append_to_selector() calls by introducing prepend_to_selector()

[felixarntz]

Backport of WordPress/gutenberg#50266

Trac ticket: https://core.trac.wordpress.org/ticket/58193

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[oandregal]

Hi Felix, would you be able to send this PR to Gutenberg? That's where the source of truth for this code lives.

This makes sense from a "code wise" perspective to me. It'd be good to engage the original authors/reviewers for help as well, so the pattern is more widely shared.

[felixarntz]

@oandregal Makes sense. I just opened WordPress/gutenberg#50266 :)

[felixarntz]

Closing this in favor of WordPress/gutenberg#50266

[tellthemachines]

Hi! I see the respective change in Gutenberg has been merged, but unless I'm mistaken this PR will still be needed to make the update in core? Shall we reopen it?

[felixarntz]

Thanks @tellthemachines, good point. I thought this was gonna be backported together with #4412, but I've reopened this PR and updated it against trunk as well as the minor changes in order to match the Gutenberg implementation that was merged.

[felixarntz]

Committed in https://core.trac.wordpress.org/changeset/55950.