This moves Result and Error to mdbook-core with the anticipation of
using them in user crates. For now, the internal APIs will be using
anyhow directly, but the intent is to transition more of these to
mdbook-core where it makes sense.
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.
