Add `completion.list.selection.auto_accept`

[miocore]

Feature Description

Is it possible to 'accept' the suggestion when i press any key while some item is selected?
basically i use workflow where i dont press enter for accepting, only select next/prev and keep typing.
would be nice to have this functionality with neovim too

[Saghen]

It's possible we need #487 for this and would likely use the same mechanism as #536