Allow to use codicons for the gutter decorations

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> > **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](https://github.com/BetterThanTomorrow/calva) 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: <img width="394" alt="image" src="https://user-images.githubusercontent.com/30010/155374881-277402fe-8766-431a-868e-cb68e41d215e.png"> This works, but [there is a need for making PENDING much clearer](https://github.com/BetterThanTomorrow/calva/issues/1543). 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.