These are two new crates intended to support implementing preprocessors
and renderers. Currently these stubs just have MDBOOK_VERSION, but
future commits will migrate more code to these crates.
This is intended as a shared, internal library that will be used by
other mdbook crates. The intention is that those crates will either
directly use, or reexport items from this crate.
Initially this includes MDBOOK_VERSION, which will get reexported from
the preprocessor and renderer crates.