silver-searcher: add alias "ag"

[dancek]

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

As discussed in #14868, ag should be an alias for silver-searcher. Rationale: ag is the command.

[jagajaga]

cc @oxij

[adnelson]

LGTM, I definitely ran nix-env -iA nixpkgs.ag before I tried silver-searcher :)