This patch for project.el adds numeric prefix buffer naming to the
project-shell and project-eshell commands, aligning their behavior with
that of eshell.

Without any prefix, these commands switch to or create a shell session.
With a plain C-u, the current behavior is to create a new session with
an automatically generated numeric suffix (e.g. "*foo-shell<2>*"), but
after switching away from that buffer, there’s no convenient way to
switch back.

This patch changes that: when given a numeric prefix (e.g. C-2), the
command will switch to or create the session buffer with the
corresponding suffix.

Implementation notes:

- project-eshell: The current implementation duplicates some
functionality provided by eshell.  By removing this redundancy, the
numeric prefix behavior is inherited "for free".

- project-shell: The revised implementation borrows from that of eshell.

Any comments or feedback would be welcome.

Thanks, best,

Paul