- Rework the `rewrite` function to spit back out Markdown instead of
  going all the way to HTML.
- Implement the `Preprocessor` type and wire up a CLI conforming to the
  mdBook preprocessor spec.
- Add relevant deps, including for an integration test.

This kind of thing works now:

```markdown
> ## Something Big
>
> Here is the content!
```

The book does a lot of that (not just `> Note: …`). 😅

Trying to make changes to this made my head hurt: the fundamental split
is on the state, *not* on the tag. So flip it.

In a scenario like this:

```markdown
> This is a quote.

> Note: this is a note
```

The result should be this output:

```markdown
> This is a quote.

<section class="note" aria-label="Note" aria-role="note">

Note: this is a note

</section>
```

The same goes for heading-type notes, and for the inverted flow.

My inspiration for these included the `aria-label` because the notes did
not have a label like `Note: ` or a heading on them; these do, though.
Keep the `aria-role` so screen readers present notes distinctively the
same way the visual presentation is set off.

Fix README: remove `aria-label` from there as well

In the destination, this will be part of a Cargo workspace, and should
*not* be in the root, where a merge will conflict with the workspace
Cargo config etc.

This will let us pull in, and work directly on, things like the
`mdbook-trpl-notes` preprocessor.

I developed this in its own repo before we knew how we wanted to
integrate it. We decided on a workspace, so this pulls it in with all
its development history.

Use the newly-introduced `mdbook-trpl-note` to render accessible and
semantic HTML for the notes/callouts in the book. This currently applies
to the nostarch version as well, so a follow-on change will update the
build process to undo that. Add CSS so the new note rendering matches
the existing rendering of blockquotes. (We might want to change that at
some point, but for now the goal is to maximizing continuity.)

Introduce a `book.toml` in the `nostarch` directory, which matches the
existing `book.toml` except for specifying the correct relative input
and output directories and disabling the `trpl-note` preprocessor. This
also lets us simplify the invocation in the `nostarch.sh` script a bit,
by providing the configuration directly in the file.

- Add building `mdbook-trpl-note` to the relevant workflow.
- Explicitly exclude `linkcheck` from new workspace.