Add `nix' project type

[matthewbauer]

I've been using this personally for a while but I think it makes sense to go in Projectile. This just registers "nix" projects as anything containing a "default.nix" file. "nix-build" actually both compiles and tests (if doCheck=true), should it be set for both?

Replace this placeholder text with a summary of the changes in your PR.
The more detailed you are, the better.

---

Before submitting a PR make sure the following things have been done (and denote this
by checking the relevant checkboxes):

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (make test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)

Thanks!

[bbatsov]

"nix-build" actually both compiles and tests (if doCheck=true), should it be set for both?

I'd say - yes.

You should also mention this in the changelog.

[matthewbauer]

Ok, looks like it's mergeable now. /cc @bbatsov