Nushell has the option to source from the login.nu file in the case
that nushell is used as a login shell. This commit adds the login file
alongside the existing config and env files as another configuration
option.
Previously the nushell module did not differentiate between Linux and
Darwin when deciding where to place config files, whereas nushell
does. This commit fixes that.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
