make the themes a module (nushell#648)

related to
- nushell/nupm#33

## description
with nushell/nupm#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
```

package.nuon

@@ -5,4 +5,5 @@
     license: "https://github.com/nushell/nu_scripts/blob/main/LICENSE"
     version: 0.1.0
     type: "module"
+    modules: ["./themes/nu-themes/"]
 }