Allow to use codicons for the gutter decorations

[PEZ]

As an extension author
I want to use the built in codicons to decorate the gutter with the same convenience as I can use codicons in the status bar
So that the user will have a consistent UI also for gutter icons (and so that I will not have to work hard to make it so)

With ”the same convenience” I mean the super convenience with strings like '$(zap)' and '$(sync~spin)'. I realize this particular API might not be suitable, but anyway.

My use case:

In my extension the user can evaluate any code in the editor. I am decorating the code being evaluated with a transparent background and displaying the result using an :after decoration. The background decoration changes color depending on the state of the operation: PENDING => white, DONE => GREEN, ERROR => red. Looks like so:

This works, but there is a need for making PENDING much clearer. I am looking at the gutter and think I would like to have a spinner there while PENDING, and I could have a checkmark when DONE and a cross when ERROR. Possibly using the same colors as I do for the marking the code being evaluated.