Nushell support for Global packages

[omerxx]

Summary

To load global packages with Nushell the user is required to eval $(devbox global shellenv). Nushell doesn't have an eval equivalent and we need to:

1. Convert the output
2. Do this on the fly when the environment is loaded
3. Maintain previous behavior

How was it tested?

Locally with Nushell based setup trying to load both bash and nu scripts.

Community Contribution License

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