Add a FindVCSRoot helper function

Quite a few tools (see #5039) are searching for `.git` to determine the project root, often as a fallback method. While git is the prevailing version control system today, it is not the only existing one in use. Thus it would make sense to search also for those other ones.

The Wikipedia article on [Version control](https://en.wikipedia.org/wiki/Version_control) could be considered to list, in the box down by the end, all notable systems to consider adding support for.

Practically I envision adding a `FindVCSRoot()` which linters may use instead of `FindNearestFileOrDirectory()`. Presumably the new function fits well in `autoload/ale/path.vim` and it would take a buffer as its only argument.

In addition to support for more version control system, this new function might also make it possible to add a `{b,g}:ale_vcs_roots = 1` configuration which some users might want to set to 0 in some cases to disable the functionality globally or for certain filetypes.