Teach ffap to look for relative file names under project-root

Teach ffap to look for relative file names under project-root

Now file-name-at-point-functions, which runs
ffap-guess-file-name-at-point, will pick up on a file name at point if
that file name is a relative file name which exists when looked up
from the root of the project.

For example, in test/lisp/progmodes/eglot-tests.el there is the string
lisp/progmodes/eglot.el; if you put point on that and C-x C-f,
lisp/progmodes/eglot.el under the root of the Emacs repo will now be
part of future history.

* lisp/ffap.el (ffap-alist): Add entry for ffap-in-project.
(ffap-in-project): Add. (bug#66668)