[WIP] Stabilize experimental Typography block support features within the block type class

[andrewserong]

Trac ticket: https://core.trac.wordpress.org/ticket/61728

🚧 🚧 🚧 WIP: This is currently a work in progress and is not yet ready for review as the Gutenberg PR has not been merged yet. 🚧 🚧 🚧

This PR syncs the PHP changes from WordPress/gutenberg#63401, however uses a slightly different approach for applying the transformations so that we do not depend on filters. The overall goal is the following:

Stabilize the following typography block supports by no longer requiring the __experimental prefix in supports.typography in block.json and when registering block types:

1. __experimentalFontFamily → fontFamily
2. __experimentalFontStyle → fontStyle
3. __experimentalFontWeight → fontWeight
4. __experimentalLetterSpacing → letterSpacing
5. __experimentalTextDecoration → textDecoration
6. __experimentalTextTransform → textTransform

The proposal here is to transform the __experimental form of the typography supports marks for stabilization when props are set on a block type instance. This occurs during registration or when a WP_Block_Type object is instantiated. There was some discussion in an earlier Gutenberg PR about potentially performing this transformation in the block registry instead — however I couldn't find a good precedent for it, and it seems slightly more encapsulated to me to keep the transformation close to where the supports key is stored.

I'm very happy for feedback on this, though!

For full testing instructions, please see the original Gutenberg PR: WordPress/gutenberg#63401

The short version is that all the typography block supports for static blocks such as Group or dynamic blocks such as Post Title should work just the same with this PR applied as without. Then, in follow-up work we can look at updating core blocks to use the non __experimental syntax.

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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.

[andrewserong]

This code intentionally does not unset the experimental version of the supports. This is so that the un-updated JS code in the editor can handle the experimental syntax, and (for now at least) feels slightly safer in terms of backwards compatibility.

[github-actions]

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[andrewserong]

Based on discussion over in WordPress/gutenberg#63401 (comment), I've updated this to move the stabilization to after the filter is run. This ensures that any opt-outs, etc, performed by plugins are appropriately stabilized before the props are set.

I've also moved the logic into a private method so that set_props is easier to read.

[github-actions]

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Props andrewserong, aaronrobertshaw.

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[aaronrobertshaw]

The latest plan is to backport all the block support stabilization updates in a single PR. This will include stabilizing border block supports and shared experimental flags such as __experimentalDefaultControls and __experimentalSkipSerialization.

More details can be found in WordPress/gutenberg#66918 (comment).

In the near future, I'll pick up this backport and update it accordingly, while @andrewserong is away.