Allow spaces after @ in CD environment, as in real LaTeX. (mathjax/MathJax#3425)

[dpvc]

This PR allows spaces after @ in the CD environment, and adds a test for that.

Resolves issue mathjax/MathJax#3425.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.72%. Comparing base (afaf1ea) to head (90d4aff).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1343   +/-   ##
========================================
  Coverage    86.72%   86.72%           
========================================
  Files          337      337           
  Lines        84145    84147    +2     
  Branches      3140     3140           
========================================
+ Hits         72971    72973    +2     
  Misses       11174    11174           

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