[CSS Gap Decorations] Ability to draw dots instead of lines

[captainbrosset]

I got this feedback after talking about CSS Gap Decorations at SmashingConf Freiburg last week.
The use case is to draw a little symbol instead of a line, as a gap decoration.

For example:

I think this would be best achieved by allowing images as gap decorations (which I know is something we plan to work on in the next iteration), but I still wanted to send the feedback, so the specific use case is taken into consideration.

In this case, the dot would need to be centered, and always the same size, even if used in other layout containers that may have different heights.