Add the tool "nixos-typecheck" that can check an option declaration to:

 - Enforce that an option declaration has a "defaultText" if and only if the
   type of the option derives from "package", "packageSet" or "nixpkgsConfig"
   and if a "default" attribute is defined.

 - Enforce that the value of the "example" attribute is wrapped with "literalExample"
   if the type of the option derives from "package", "packageSet" or "nixpkgsConfig".

 - Warn if a "defaultText" is defined in an option declaration if the type of
   the option does not derive from "package", "packageSet" or "nixpkgsConfig".

 - Warn if no "type" is defined in an option declaration.