infra: share infrastructure for mdbook preprocessors

- Create an `mdbook_trpl` library package which hosts shared concerns
  for the packages, e.g. error and config handling.
- Move `mdbook-trpl-note` and `mdbook-trpl-listing` into the new shared
  package, with binaries at `src/bin/(note|listing)/main.rs` and the
  existing libraries at `src/note/mod.rs` and `src/listing/mod.rs` with
  their associated tests.
- Extract their actual shared pieces into the crate root.

At the moment, this doesn't do a lot except trim down the number of
packages in the repository, but it sets things up nicely to support more
preprocessors (which I am going to add shortly).