Provide completions for Nushell

[ysthakur]

This PR provides completions for z and zi. Nushell no longer uses external completers for internal commands since v0.103 (release notes), so people's old external Zoxide completers won't work anymore.

This completer uses the option completion_algorithm: "substring", which was only added in 0.104. An alternative would be to use completion_algorithm: "prefix", positional: false, which works in older versions but is deprecated.

This completer also looks a little weird if you type in multiple arguments. If you type in z /foo bar<TAB>, all the completions you see will have everything up to and including the first /foo stripped. It's not great, but it's needed because custom completers in Nushell can't currently choose which span they want to replace. Without it, if your first result upon typing z /foo bar<TAB> were /foo/bar, hitting Enter would replace your command line with z /foo /foo/bar, which is incorrect.