#52569 Create post-terms block variations for custom taxonomies in editor

[gaambo]

What?

This is a first try at fixing #52569.
I try to create the variations of the post-terms block for all custom taxonomies in the editor instead of on the server side.

Why?

See #52569 - server side function runs too early to make sure it gets all custom taxonomies by plugins, themes etc.

How?

I have to get the taxonomies from the rest api - and the results of them may come in async - therefore I have to use subscribe and unsubscribe. That's the only way I know right now how to get this kind of data (post types, taxonomies) on the "start" of the editor outside of react. If anyone has a better approach, I'm very happy to change this.
Therefore I'm using registerBlockVariation as soon as the data is available and I can't use the blocks.registerBlockType filter as in the previous code.

Some caveats:

• Because of the REST API call, this only gets taxonomies which have "show_in_rest" enabled.
• Maybe we can preload all the taxonomies data (like the navigation block does), but that can be a lot of data on large sites.
• I don't know of the performance impacts of subscribe/unsubscribe but could imagine it's kind of large because
• The _builtin property of taxonomies is not output via REST API. But by default the builtin taxonomies are registered first and therefore output first (and therefore added as a variation first).

Testing Instructions

1. Register a custom taxonomy. (See explanation below)
2. Create a new page/post.
3. See that the blocks for builtin taxonomies "Tags", "Categories" are available
4. See that a block "Courses" is available
5. Check all three blocks are displayed with the correct name and can be switch with the variation switcher.

How to create custom taxonomy:
I've used the example code from developer.wordpress.org and put it in a simple plugin which I have mapped to load into gutenberg.

in .wp-env.json add:

	"mappings": {
		"wp-content/plugins/test-taxonomy": "../test-taxonomy"
	},

In this case i've added the sample plugin in a directory named "test-taxonomy" one level above the gutenberg directory.

TODO:

• Maybe set one variation as the default so the "post terms" "naked" variation does not show
• Maybe only fetch taxonomies that are releated to the currently edited post type (eg on custom post types, tags & categories would not be shown).

Screenshots or screencast

[gaambo]

The approach in #54801 to use the registered_taxonomy seems promising, therefore probably going with this approach. If #54801 is merged, we can do the same for the post-terms block.