bug#54296: Add buffer-matching functionality

[Dmitry Gutov]

On 14.03.2022 05:21, Eli Zaretskii wrote:
> I still don't think I understand why it is so
> important that we should provide a special case for it.

It's pretty valuable to be able to provide a default value of 
project-kill-buffer-conditions which can be read at a glance (including 
in compiled Emacs) and easily edited via customize-variable by a user 
with little to no knowledge of Elisp.

And 'major-mode' was used twice inside project-kill-buffer-conditions, 
not just once.