Provide jump commands completion for nushell

[alecandido]

For custom commands (defined in Nushell itself), completion has to be directly attached to the command definition
https://www.nushell.sh/book/custom_completions.html#custom-completions

So, better to provide completion in the template itself :)

---

Notice that there is an entire section in the official docs of Nushell discussing zoxide completion
https://www.nushell.sh/cookbook/external_completers.html#zoxide-completer

However,

This completer is not usable for almost every other command, so it's recommended to add it as an override in the multiple completer:
[...]
Sometimes, a single external completer is not flexible enough.

i.e., it always assumes that zoxide commands are provided as external ones. While the Nushell template is defining __zoxide_z and __zoxide_zi as custom commands (internal ones).