Tentative Vim support for virtualtext

[AndrewRadev]

Upstream Vim got some initial support for virtual text attached to text props: https://vimhelp.org/textprop.txt.html#virtual-text. It's likely there's bugs and edge cases, and I can't see a way to attach padding, but it seems functional, at least:

This PR is based on top of #10 since that fixes an issue for me.