Expose the table structure attributes in the struct tree

[calixteman]

Preserve RowSpan, ColSpan, Headers, Scope, Short and Summary when serializing the structure tree, and map them to the corresponding ARIA attributes in the viewer.

The attributes are now collected from both the A and the C entries, which may be arrays interleaving dictionaries and revision numbers: this also fixes the bounding box and the MS Office MathML lookups, which only handled a single direct dictionary.

Fixes #18090.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.77419% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.04%. Comparing base (5ca8134) to head (240bfee).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/core/struct_tree.js	96.66%	3 Missing ⚠️
web/struct_tree_layer_builder.js	96.92%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #21702    +/-   ##
========================================
  Coverage   90.04%   90.04%            
========================================
  Files         264      264            
  Lines       66899    67037   +138     
========================================
+ Hits        60237    60365   +128     
- Misses       6662     6672    +10     

Flag	Coverage Δ
browsertest	66.35% <0.64%> (-0.14%)	⬇️
fonttest	9.03% <ø> (ø)
integrationtest	69.34% <69.67%> (-0.03%)	⬇️
unittest	58.09% <94.83%> (+0.20%)	⬆️
unittestcli	56.64% <56.12%> (+0.01%)	⬆️

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[timvandermeij]

r=me, with one optional comment. Thanks!