fix: streamline with vim.ui.select; add stylua

[fdschmidt93]

Streamlines a few things:

• opts or its keys may be nil -- we now default to what vim.ui.select does
• on_choice should accept idx like vim.ui.select
• added stylua from core bc I was confused what happened when I formatted otherwise :laughing: -- let me know if you prefer that in a separate commit

[Conni2461]

You can add it. I only "upstreamed" stuff i had in my config.

But .stylua.toml is not part of the commit.

[fdschmidt93]

Helps if I'd also track new files. Done.

[Conni2461]

thanks :)