feat(experimental): fix tab completions

[machow]

• Wraps the _jedi_matches function from IPython, rather than vendoring it.
• Now sorts results so column names are first.

TODO: validate this works on vscode (from pairing, it looks like it doesn't) works, see comment below

[machow]

Ah, @isabelizimm uncovered that you can use these settings in vscode to enable:

• "python.languageServer": "Jedi"
• "jupyter.enableExtendedKernelCompletions": true