update nushell init script

[Bahex]

1. Made __zoxide_z more idiomatic
2. Updated the hook initialization to avoid breakage on nushell's future 0.102.0 release

To expand on hook initialization:
After nushell/nushell#14704 (comment), starting sub shells would make zoxide's init script throw an error.

With this PR $env.__zoxide_hooked is no longer used. Instead, the list of hooks are checked directly to see if zoxide's hook has already been added.

I checked the new init script on both nushell version 0.89.0 and the latest nushell nightly.
Minimum supported version of nushell doesn't need to change and can be kept at 0.89.0