bug#54296: Add buffer-matching functionality

[martin rudalics]

> This seems to work, and updating window.el was also pretty trivial
> (unless I have missed something):

IIUC the doc-string would now say that

  This returns the cdr of the alist entry ALIST if either its key
  satisfied a BUFFER-NAME per `buffer-match'.

which doesn't appear correct.  In either case please make sure that your
change does not affect existing behavior where a buffer is not displayed
in a specific way because it does not match 'display-buffer-assq-regexp'
until now.

Thanks, martin