Show selected popupmenu item using virtual text

[famiu]

Problem

'completeopt' without noinsert can be way too intrusive as you can't easily add characters to the completion prefix. But 'completeopt' with noinsert is also not optimal as it actually inserts text into the buffer when an item except the first one is selected. Moreover, noinsert makes it harder to preview the result of picking the first completion item without inserting it unless you do something like <C-n><C-p>.

Expected behavior

It is possible to leverage virtual text for this. We can either:

• Show virtual text for noinsert instead of inserting the text. OR,
• Add a virttext flag (tentatively named) to completeopt, which will show the selected item using virtual text.