Heading: Enable heading level curation via theme.json

[vyskoczilova]

This PR #63535 enabled setting Heading levels via PHP snippet when registering the block. I would love to see an option to define those in the theme.json, like you can limit color settings, etc.

This would allow easier maintenance and the possibility of changing this later (for example, for certain post types) with the filter wp_theme_json_data_theme.

[Mamaduka]

I'm not sure if theme.json is the right place for this; it controls the theme's global settings and styles. The levelOptions attribute, on the other hand, is a block-specific setting that can be overridden on an instance level.

cc @fabiankaegy, @ndiego

[vyskoczilova]

For me, it goes the same as allowing the disabling of some specific features, and at the same time, it will be easy to access the feature. The point is that there is no way (or at least I haven't found it) to modify levels based on CPTs. Let's say that for posts, they decided only to use H2 and H3, not smaller ones, but on standard pages, they need even smaller headlines.