Problem: Plugins may want to have a way to show more details about items
when using `vim.ui.select`. This is a fairly common problem that
prompts plugin authors to implement dedicated sources/pickers for
fuzzy picker plugins that are popular at the moment.
Solution: Document a way for `vim.ui.select` to provide preview:
- `vim.ui.select` users can provide `opts.preview_item` function that
creates/uses a buffer and its contents at certain position to show
more details about an item.
- `vim.ui.select` implementations may use `opts.preview_item` in the
way they see fit (like show the buffer in a separate/same window
interactively/on-demand or do nothing) if they have a way to show
more information about an item.
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