Add dir=ltr to MathJax container element. (mathjax/MathJax#3589)

[dpvc]

This PR adds a dir="ltr" attribute to the mjx-container element so that the math remains left-to-right when used in right-to-left contexts. The SVG output had CSS direction: ltr, but apparently this by itself isn't enough to override q right-to-left direction. Adding unicode-bidi: isolate would do it, but apparently dir="ltr" is to be preferred (see the warning on this MDN page).

Resolves issue mathjax/MathJax#3589.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.47%. Comparing base (aa26ce0) to head (5739b06).

Files with missing lines	Patch %	Lines
ts/output/common.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1526      +/-   ##
===========================================
- Coverage    86.47%   86.47%   -0.01%     
===========================================
  Files          340      340              
  Lines        86178    86177       -1     
  Branches      4856     4856              
===========================================
- Hits         74520    74519       -1     
  Misses       11637    11637              
  Partials        21       21              

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