Expose more things in the public API

[chrisjsewell]

As discussed in https://github.com/executablebooks/rst-to-myst/pull/18#issuecomment-863875739:

I think we may want to work on making something like build_mdit public to alleviate the various gotchas involved (regarding plugins and default conf options for example).

Also things in mdformat/renderer/_util.py, e.g. mdformat-myst currently duplicates longest_consecutive_sequence.