Add nixpkgs support to doctor command.

[ianmjones]

Resolves #1550

Now wails doctor works on NixOS:

ian@ians-tuxedo:~ $ wails doctor
Wails CLI v2.0.0-beta.38

Scanning system - Please wait (this may take a long time)...Done.

System
------
OS:             NixOS
Version:        22.11
ID:             nixos
Go Version:     go1.17.11
Platform:       linux
Architecture:   amd64

Wails
------
Version:                v2.0.0-beta.38
Package Manager:        nixpkgs

Dependency      Package Name    Status          Version
----------      ------------    ------          -------
*docker         nixos.docker    Installed       20.10.17
gcc             nixos.gcc       Installed       11.3.0
libgtk-3        nixos.gtk3      Installed       3.24.34
libwebkit       nixos.webkitgtk Installed       2.36.4
npm             nixos.nodejs    Installed       16.15.0
*nsis           nixos.nsis      Available       3.06.1
pkg-config      nixos.pkg-configInstalled       0.29.2
*upx            nixos.upx       Installed       3.96

* - Optional Dependency

Diagnosis
---------
Your system is ready for Wails development!
Optional package(s) installation details:
  - nsis: nix-env -iA nixos.nsis

I also fixed typos related to the word "dependency".

[leaanthony]

Nice one! Thanks for doing this!