Add integration with project.el

[non-Jedi]

This code is already used by eglot-jl to locate Julia projects and
will be useful in the future for other Julia packages (such as for
REPLs to decide which files they're associated with). Putting it in
the julia-mode package means it doesn't have to be redefined each time
it's used.

[non-Jedi]

This doesn't byte-compile cleanly on Emacs 24 since cl-defmethod was added in 25.1, however all tests pass since that section is never executed since the 'project feature was also added in 25.1. I'm sure there's a way to work around this, but I don't know how off the top of my head. Using defmethod instead of cl-defmethod isn't flexible enough in this case.

[tpapp]

I am wondering if any package other than eglot-jl uses/needs something like this.

Also, I am reluctant to add a feature that only works on Emacs 25+, since we still support Emacs 24.

[non-Jedi]

I found myself wanting this while working on #106, which is the reason I opened it. Still might not be worthwhile if trying to support Emacs 24.

[berquist]

Can this be merged now that the minimum required Emacs version is 26.1?

[non-Jedi]

@berquist What's your use-case for having this code in julia-mode?