bug#66260: 29.0.92; project.el should support copying a file path relative to the project root

[sbaugh]

When communicating with others, it is often useful to copy a file path
to the current file which is relative to the project root.  For example
I'd rather send someone the path lisp/progmodes/project.el than
~/src/emacs/trunk/lisp/progmodes/project.el.

project.el should have a way to copy this file path.

Almost good enough is:

 C-x p f       ;; project-find-file
 M-n           ;; next-history-element
 C-k           ;; kill-line

but with project--read-file-cpd-relative, if all the files in
project-files start with a common-parent-directory which is a
subdirectory of project-root, the resulting file path will not be
relative to the project-root but to the subdirectory.

Maybe the thing to do is just make project--read-file-cpd-relative use
project-root instead of calculating a common-parent-directory?  I would
be happy to make that patch if that's acceptable.  I think that's
probably the best option.

Alternatively, it might be cool if there was a variation on:

 C-x C-j     ;; dired-jump
 M-0         ;; digit-argument
 w           ;; dired-copy-filename-as-kill

(which copies an absolute path) to instead copy a project-root-relative
path.


In GNU Emacs 29.0.92 (build 68, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw3d scroll bars) of 2023-07-15 built on earth
Repository revision: 36d3460f9f6064c03fd88e0c969c8e4f4d645235
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: NixOS 23.05 (Stoat)

Configured using:
 'configure --cache-file=config.cache --with-x-toolkit=lucid
 --with-tree-sitter --with-xinput2 CC=gcc PKG_CONFIG=pkg-config
 
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Load-path shadows:
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