feat(?): synax highliting in popup with snippet of completion source (also rendeting bug repot)

[msva]

Hi there!

1. it would be great, if code would be syntax-highlited with for appropriate language in the popup windows with snipped of code where completion was taken (although, I'm not sure, if it worth, as there are ust few lines usually 🤷). Now it renders with no colors, see screenshot.
   Also, actually, I'm not sure if it is not cmp itself issue 🤷

2. there is a glitch in rendering the "frames" of the popup with source snippet, if there is long lines there (see screenshot, notice right popup corner)

Although, I also not sure it it is not a "cmp" itself issue here, actually 😀

[msva]

(also, it would be nice if background of that popup will be inherited from cmp's menu)

[lukas-reineke]

1 shouldn't be too hard, I'll look into this.

2 and the background color are both issues with cmp itself, please open an issue in the cmp repo.