Improve MathML for \not

[ronkok]

This PR resolves a long-standing TODO item by implementing \not in MathML. The code employed is similar to the code used for HTML \not prior to PR #1497.

[codecov-io]

Codecov Report

Merging #1923 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1923      +/-   ##
==========================================
+ Coverage   94.82%   94.84%   +0.01%     
==========================================
  Files          79       79              
  Lines        4776     4788      +12     
  Branches      842      847       +5     
==========================================
+ Hits         4529     4541      +12     
  Misses        223      223              
  Partials       24       24

Flag	Coverage Δ
#screenshotter	89.34% <100%> (+0.2%)	⬆️
#test	85.79% <46.15%> (-0.12%)	⬇️

Impacted Files	Coverage Δ
src/macros.js	97.2% <100%> (ø)	⬆️
src/buildMathML.js	98.9% <100%> (+0.16%)	⬆️

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[codecov-io]

Codecov Report

Merging #1923 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1923      +/-   ##
==========================================
+ Coverage   94.82%   94.83%   +<.01%     
==========================================
  Files          79       79              
  Lines        4776     4783       +7     
  Branches      842      845       +3     
==========================================
+ Hits         4529     4536       +7     
  Misses        223      223              
  Partials       24       24

Flag	Coverage Δ
#screenshotter	89.33% <100%> (+0.18%)	⬆️
#test	85.84% <50%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
src/macros.js	97.2% <100%> (ø)	⬆️
src/buildMathML.js	98.83% <100%> (+0.1%)	⬆️

---

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[ylemkimon]

I think we can combine with the loop above, looping only once with the same or less number of checks:

            // Combine \not with mrels and mords
/*L#148*/   } else if (lastGroup.type === 'mi' && lastGroup.children.length === 1) {
                const lastChild = lastGroup.children[0];
                if (lastChild instanceof TextNode && lastChild.text === '\u0338' &&
                    (group.type === 'mo' || group.type === 'mi' || group.type === 'mn')) {
                    const child = group.children[0];
                    if (child instanceof TextNode && child.text.length > 0) {
                        // Overlay with combining character long solidus
                        child.text = child.text.slice(0, 1) + "\u0338" + child.text.slice(1);
                        groups.pop();
                    }
                }
            }

[ronkok]

Done.