[Finishes #71905084] dont highlight blocks while dragging

Turns out this was a regression of mine.  In Maze.performStep I set traceOn(true) at the start of each step.  This was done so that if you clicked off a block, we would still go back to highlighting on the next step.  Previous to that, there wasn't this same interaction between dragging and highlighting, because you would start the drag, it would change the selection, it would set traceOn(false), and we would stop highlighting blocks.

When blockly-core highlights a block, it sets that block to be the selection.  When the selection changes during a drag (something that became possible when I turned trace back on for every step), things blow up.  Fix is to disable block highlighting while dragging.