Inconsistent appearances of number scrubber

The number scrubber doesn't appear for all numbers, especially if there are certain combinations of letters symbols, or (lack of?) spaces that precede it.

I've documented a whole bunch of possible combinations here. Test by clicking every occurrence of "123" (or scrolling through with arrow keys).
https://www.khanacademy.org/computer-programming/number-scrubber-bug/5383506645876736

It's a little unclear if we want to prevent scrubbing of numbers that are part of variable names, so we should probably settle that before attempting to make a fix (if needed).

Start with the linked number-scrubber example and reproduce the documented combinations by clicking or navigating through each occurrence of "123". First settle whether numbers inside variable names should be scrubbed; done means the agreed behavior is consistent across the documented cases.

Written by the indexing model from the issue text.