check Nushell files in the CI (dragoon/komodo!137)

in order to help catch issues with the various Nushell scripts and modules in the source base, i propose to add a CI script that will check them all.

below is an example error, e.g. when introducing a syntax error in `.env.nu`:
```
Error:   × Failed to parse content:
  │     file: .env.nu
  │     err:  Expected keyword.
```