Please add nushell completion

**What would you like to be added**:
kubectl completion nushell

**Why is this needed**:
Nushell is amazing for working with kubernetes, exceeding every other shell i know. The only disadvantage is the lack of completion.

Start from the `kubectl completion nushell` command and locate the existing completion entry points for other shells. The issue names no files or tests; done means the command emits usable Nushell completion output, with behavior covered consistently with the existing completion commands.

Written by the indexing model from the issue text.