Added check and message for X being misused as *

Submitting this as a result of @spicyj's feedback from https://github.com/Khan/khan-exercises/pull/71829

The code's a little more complicated compared to the khan-exercises pull request thanks to some information that's not accessible in KAS (like the original input string and which side is the "solution").

A way to get back to that simpler implementation would be to just have a KAS.xUsedAsMultOp function that takes the parameters the old function took, but to me it looks like the "right thing" is to just have KAS.compare spit out a new message.

N.B.: There are two checks, one after sameVars failure, another after expr.compare failure.  This is because expressions that match the check can go down either path.