Fix issue #33473, allow white space in expressions involving subtraction

13-2v or 13 -2v was being accepted, but 13 - 2v was not.
https://github.com/Khan/khan-exercises/issues/33473
Allow whitespace after negation and subtraction.

Add \\s* after all instances of [-\\u2212] in exercises and tested.
There is no reason for counting students wrong for variance in formatting
of this type, and it is not infrequent to see expressions as 5 - x in text.
var tag incorrectly terminated with a /span ending tag

This was causing errors on our local school server by
students with Macs using Safari.
Merge pull request #33495 from saasmath/fix_issue_33473

Fix #33473, allow white space in expressions involving subtraction