Ignore braces in array preamble. (mathjax/MathJax#3565)

[dpvc]

This PR adds rules to the array preamble column parser to ignore braces within the preamble (when they aren't used to enclose an argument to one of the column specifications). This isn't exactly in line with LaTeX, which does seem to process the braces in some way, but I can't figure out what it is, and don't see any documentation for it. It looks like it might be to allow multi-character column specifiers, but then it also seem to allow multiple copies of a single column specifier, but not mixed ones. So I'm not sure what it actually does. In any case, ignoring them seems to handle the situation given in the original issue tracker.

I also added test for this situation.

Resolves issue mathjax/MathJax#3565.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.47%. Comparing base (a65456c) to head (5dc3abe).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1508   +/-   ##
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  Coverage    86.47%   86.47%           
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  Files          340      340           
  Lines        86125    86127    +2     
  Branches      3206     3206           
========================================
+ Hits         74476    74478    +2     
  Misses       11649    11649           

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