Make the keep-attrs attribute into a property, since it is removed after use

[dpvc]

This PR changes the mjx-keep-attrs attribute into a property, since it is only used in the tex input phase, and doesn't need to be exposed as an explicit attribute. It also removes the unused _d argument from the walkTree() call.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.66%. Comparing base (27c47f3) to head (f5429ea).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1379      +/-   ##
===========================================
- Coverage    86.66%   86.66%   -0.01%     
===========================================
  Files          338      338              
  Lines        84384    84382       -2     
  Branches      4779     3148    -1631     
===========================================
- Hits         73131    73129       -2     
  Misses       11253    11253              

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[zorkow]

lgtm.