This removes the file nix-mode.el from Nix. The file is now available within the
repository https://github.com/NixOS/nix-mode.

Fixes #662
Fixes #1040
Fixes #1054
Fixes #1055
Closes #1119
Fixes #1419

NOTE: all of the above should be fixed within NixOS/nix-mode. If one of those
hasn’t please reopen within NixOS/nix-mode and not within NixOS/nix.

(cherry picked from commit c20641ce)

(cherry picked from commit 0fb60e4e)

Looks like this snuck into the 1.11 release notes post-release, but
float support isn't actually present until 1.12.

We set build-cores automatically, not build-max-jobs. (The commit
message for de4cdd0d also got this
wrong.)

- rename options but leav old names as lower-priority aliases,
  also "-dirs" -> "-paths" to get closer to the meaning
- update docs to reflect the new names (old aliases are not documented),
  including a new file with release notes
- tests need an update after corresponding changes to nixpkgs
- __noChroot is left as it is (after discussion on the PR)

So you can now do things like:

  $ nix-env -qa '.*zip.*'
  $ nix-env -qa '.*(firefox|chromium).*'