Flatten a bunch of non-pervasive ParseNode types (part 2)

[marcianx]

Towards #1492.

[codecov]

Codecov Report

Merging #1552 into master will increase coverage by 0.01%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master    #1552      +/-   ##
==========================================
+ Coverage    84.1%   84.12%   +0.01%     
==========================================
  Files          79       79              
  Lines        4555     4554       -1     
  Branches      803      803              
==========================================
  Hits         3831     3831              
+ Misses        629      628       -1     
  Partials       95       95

Impacted Files	Coverage Δ
src/functions/environment.js	100% <ø> (ø)	⬆️
src/parseNode.js	84.21% <ø> (ø)	⬆️
src/utils.js	66.66% <0%> (ø)	⬆️
src/functions/underline.js	9.09% <0%> (+0.75%)	⬆️
src/buildHTML.js	97.69% <0%> (ø)	⬆️
src/functions/overline.js	25% <0%> (ø)	⬆️
src/Parser.js	93.94% <100%> (ø)	⬆️
src/functions/operatorname.js	92.1% <100%> (ø)	⬆️
src/functions/enclose.js	90.76% <100%> (ø)	⬆️
src/functions/raisebox.js	100% <100%> (ø)	⬆️
... and 9 more

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[kevinbarabash]

Yay! Thanks for doing all of these refactors. I'm glad to see that you're able to do them in chunks.

[kevinbarabash]

Much better!

[kevinbarabash]

@ylemkimon thanks for reviewing all of these large refactors.