Use `nushell` v0.63 `PWD` hook instead of custom prompt

[leroycep]

In v0.63 of nushell, they introduced environment change hooks. This means zoxide can listen for PWD changes directly:

hooks: {
    env_change: {
        PWD: [{|before, after|
            zoxide add $after
        }]
    }
}

[ajeetdsouza]

Blocked on https://github.com/nushell/nushell/issues/5951.