Properly handle trailing spaces in \def. (mathjax/MathJax#3550)

[dpvc]

This PR fixes a problem with the \def command where trailing spaces are treated incorrectly as part of a template macro definition. E.g.

\def\x#1  {[#1]}

would include the space as part of a template, so $$\x{y}$$ would fail (it would have to be $$\x{y} $$. The trailing spaces should only be included in a template if there are other template entries like

\def\x#1,#2 {[#1,#2]}

or

\def\x#1 #2 {[#1,#2]}

Resolves issue mathjax/MathJax#3550.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.45%. Comparing base (da21770) to head (08e87fa).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1455     +/-   ##
==========================================
  Coverage    86.45%   86.45%             
==========================================
  Files          340      340             
  Lines        86106    86109      +3     
  Branches      3191     4835   +1644     
==========================================
+ Hits         74439    74442      +3     
  Misses       11667    11667             

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[zorkow]

lgtm.

[zorkow]

lgtm.