Clickable interpreter debug breakpoints

[IgnoredAmbience]

We can currently set interpreter breakpoints using the S_line and I_line condition expressions.

It would be nice if there were an easier way of entering these breakpoints.

CodeMirror supports setting markers in the editor gutter using the doc.setGutterMarker, and handling clicks using the gutterClick event.

CodeMirror demo of this: https://codemirror.net/demo/marker.html