Editor: Backport hooks to filter theme.json data from Gutenberg to 6.1.
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:
Props oandregal, czapla, gziolo, bernhard-reiter.
See #56467.