related to
- https://github.com/nushell/nupm/pull/33
## description
with https://github.com/nushell/nupm/pull/33, Nupm can now install
separate modules from a single package 🥳
the `nu_scripts` are a package and looks like they contain a few
potential standalone modules:
- the themes
- the hooks
- the custom completions
- ...
this PR is a proposal to move `themes/` to `nu-themes/` and add
`./nu-themes/` as a module to the `nu-scripts` package.
that way when running `nupm install --path .` in the root of the
`nu_scripts`, a `nu-themes` module will be installed in
`$env.NUPM_HOME/modules`.
then one can run
```nushell
use nu-themes/themes/nushell-dark.nu
```
and next get the theme with
```nushell
nushell-dark
```
