This changeset ports the work done in Gutenberg (released in 14.1) to add hooks to filter the `theme.json` data. Specifically, it adds the following filters: `theme_json_default`, `theme_json_blocks`, `theme_json_theme`, and `theme_json_user`.
For more details, see the following Gutenberg pull requests:
- [https://github.com/WordPress/gutenberg/pull/44015 gutenberg#44015]: Make global styles data filterable
- [https://github.com/WordPress/gutenberg/pull/44109 gutenberg#44109]: Prepare `WP_Theme_JSON_Data` class for backporting
- [https://github.com/WordPress/gutenberg/pull/44159 gutenberg#44159]: Update hook's names from `global_styles_*` to `theme_json_*`
Props oandregal, czapla, gziolo, bernhard-reiter.
See #56467.
git-svn-id: https://develop.svn.wordpress.org/trunk@54251 602fd350-edb4-49c9-b593-d223f7449a82
