Problem: some clients like cxserver (T344982) need a way to convert between wikitext and HTML on the fly. Restbase provides the tranform API to address this need. If we deprecate RESTbase, we need a replacement.
Solution: expose the tranform API already implemented in core. That API is already accessible in development mode (in CI, on testwiki and on beta), and it is also the basis of the transformation API implemented by the parsoid extension which is what we currently expose through restbase.
NOTE: we probably don't want to enable support for the pagebundle format, just wikitext, html, and lint.