Add a new racket-pdb-mode

Recent commits explored racket-xp-mode optionally using pdb in much
the same way as before: Getting all annotations for the entire buffer
and inserting them as text properties. This was an interesting sanity
check, but aside from adding multi-file rename, didn't really offer
much value.

In this commit, make a fresh racket-pdb-mode, which fully buys into
the idea of querying only what's needed, when needed.

Upon motion (change in point and/or window start/end) we merely add
some def/use overlays for the visible portion of the window, and
racket-show any mouse-over for point.

As a result, this works much better for large files. Although we still
might need to wait 10 seconds for something like
private/class-internal.rkt to be analyzed, Emacs isn't blocked for
that, and when the analysis is ready, it's not
blocked (re)propertizing the entire buffer.