bug#48747: add project-name generic

[Stephen Leake]

eglot provides a use case:

eglot builds a name for a server using the root directory of the
project - in effect:

(file-name-base (directory-file-name (project-root (project-current))))

That name shows up in the elgot mode line, to tell the user which server
the buffer is connected to, in progress report messages, and in the name
of the EGLOT log buffer, which is useful for debugging things.

If the project root directory happens to have a meaningful name, that's
fine. In my use cases, it's usually not meaningful. For example, I have
two worktrees of my wisitoken project, one for the main branch, one for
a work branch. The eglot names, and the ones I'd like to see, are:

    default     desired
    "build"     "wisitoken main"
    "build"     "wisitoken work"

Similarly, the name for the ada_language_server worktree is:

    "gnat"      "als main"

I could override project-name that in my projects to provide my desired
name, and eglot will use my desired name.

-- 
-- Stephe