Basic idea is:
- add a core hook for looking up TranclusionData (including from a parser function or magic word)
- Move TD specification to docs/TransclusionData.md
- Add an MCR content type, TransclusionData extends Json
- Currently Extension:TemplateData will be the provider of TransclusionData using its existing mechanism, but eventually we would be able to fetch/store it in an MCR slot.
- Probably ship transclusion data for core parser functions etc (T55414)
This plan is sketched out in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/TemplateData/+/921583/1/includes/Hooks.php and mentioned in the comments to https://gerrit.wikimedia.org/r/c/mediawiki/extensions/TemplateData/+/819740.