Make sure multi-byte UTF8 characters are handled properly in data-latex attributes. (mathjax/MathJax#3575)

[dpvc]

This PR fixes a problem where unicode characters from planes other than Plane0 are not properly entered in some data-latex attributes. The problem is that for a character that requires multiple string character at the beginning of the attribute, we need to back up the correct number of characters (not just 1). The fix uses the string length of the unicode character passed to it.

Resolves issue mathjax/MathJax#3575.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.47%. Comparing base (a65456c) to head (68caffc).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1505     +/-   ##
==========================================
  Coverage    86.47%   86.47%             
==========================================
  Files          340      340             
  Lines        86125    86125             
  Branches      3206     4848   +1642     
==========================================
  Hits         74476    74476             
+ Misses       11649    11628     -21     
- Partials         0       21     +21     

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