Use `vim.ui.input` to maintain cmdheight=0 and still show prompt for char(s) input

[megalithic]

https://github.com/phaazon/hop.nvim/blob/2a1b686aad85a3c241f8cd8fd42eb09c7de5ed79/lua/hop/init.lua#L218

Related https://github.com/phaazon/hop.nvim/issues/296 and https://github.com/phaazon/hop.nvim/issues/276

[jghauser]

I just want to chime in to say that's I think this would be a good idea and that it would also be great if we could optionally turn off the prompt (because vim.input triggers a notification on my system and that would get a bit much).

If @phaazon thinks this would make sense, I could write something and make a PR.