feat: make VCS marker detection order configurable

[stribb]

Motivation

projectile-project-vcs checks VCS markers in a fixed order — .git
before .jj — so a colocated jj+git repo is always detected as git.

This is a common situation rather than an edge case: jj's most popular
backend is git. When you run jj git init in an existing repo, or
clone with jj git clone, you get a directory that contains both a
.jj/ folder and a .git/ folder. Users of jj-on-git who want
projectile to treat such repos as jj projects have no recourse short of
advising projectile-project-vcs.

Changes

Add projectile-vcs-markers — an ordered alist of (marker . vcs-symbol)
pairs — and rewrite projectile-project-vcs to iterate it for both the
root check and the dominating-file walk, replacing the hardcoded cond.

The default order is unchanged from the current behaviour, so this is
backwards-compatible for all existing users.

Usage

To prefer jj over git in colocated repos:

(setq projectile-vcs-markers
      (cons '(".jj" . jj)
            (assoc-delete-all ".jj" projectile-vcs-markers)))

Tests

Four new Buttercup specs in projectile-test.el covering:

• single .git repo → git
• single .jj repo → jj
• colocated .git+.jj with default order → git
• colocated .git+.jj with custom order (jj first) → jj

[bbatsov]

I think that's a good change. Just rebase it on top of master and we're good to go.

You might also mention your usecases and the configuration somewhere in the user docs.

[stribb]

Rebased onto master and added docs. Ready for merge.