Update and combine theme.json and block.json extractors

[swissspidy]

Uses the block-i18n.json schema file to determine which fields are translatable.

Enhancement: Adds support for translating block variation fields. Apparently these were not extracted before...

Fixes #287

[swissspidy]

cc @gziolo

[swissspidy]

Looks like the existing theme i18n schema parser does not support cases like

  "keywords": [ "block keyword" ],

as seen in block.json

So that needs some fiddling to enable support and make the tests pass

[swissspidy]

@schlessera I'm running into some fatal errors here on PHP < 7.4. Perhaps you have some ideas here? Not sure which one of my changes could have caused that...

[schlessera]

@swissspidy The problem was the use of private access in the IterableCodeExtractor trait. That worked fine until you extended a class that was using that trait.

[swissspidy]

Ah 💡 🤦 Thanks!

[gziolo]

I think it's a good idea to use the file from wordpress-develop for now. I'm still not sure whether using the canonical URL similar to https://schemas.wp.org/trunk/block.json as discussed in the parent issue would work so nicely as it can be ahead of WordPress core.

Anyway, it looks like this PR has everything covered. Nice work, thank you for bringing i18n schemas up to date ❤️