feat: \dddot and \ddddot support

[edemaine]

What is the previous behavior before this PR?
\dddot and \dddot were not supported, and requested in #2744

What is the new behavior after this PR?
Fixes #2744, though italic correction is off. Example:

x+\dddot x + \ddddot y + y

KaTeX rendering:

LaTeX rendering:

It looks much worse for a letter A.

Also missing are tests. Is this worth a screenshot test? Maybe, for texcmp purposes?

[codecov]

Codecov Report

Merging #3834 (1bbb25a) into main (5dd9bc4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #3834   +/-   ##
=======================================
  Coverage   92.98%   92.98%           
=======================================
  Files          91       91           
  Lines        6770     6772    +2     
  Branches     1574     1574           
=======================================
+ Hits         6295     6297    +2     
  Misses        437      437           
  Partials       38       38           

Impacted Files	Coverage Δ
src/macros.js	97.51% <100.00%> (+0.01%)	⬆️

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[ronkok]

I think the rendering is about as good as we can expect to get.

The PR needs documentation updates. For the accent table in supported.md, I suggest modifying the bottom few rows as follows:

|$\dot{a}$ `\dot{a}` |$\overleftrightarrow{AB}$ `\overleftrightarrow{AB}`  |$\overbrace{AB}$ `\overbrace{AB}`
|$\ddot{a}$ `\ddot{a}`  |$\underleftrightarrow{AB}$ `\underleftrightarrow{AB}`|$\underbrace{AB}$ `\underbrace{AB}`
|$\dddot{a}$ `\dddot{a}`|$\overline{AB}$ `\overline{AB}` |$\overlinesegment{AB}$ `\overlinesegment{AB}`
|$\ddddot{a}$ `\ddddot{a}`|$\underline{AB}$ `\underline{AB}`  |$\underlinesegment{AB}$ `\underlinesegment{AB}`
|$\grave{a}$ `\grave{a}`|$\widecheck{ac}$ `\widecheck{ac}`  |$\underbar{X}$ `\underbar{X}`
|$\hat{\theta}$ `\hat{\theta}`|$\widehat{ac}$ `\widehat{ac}`||

support_table.md should also be updated. It's pretty straightforward.

With those qualifications, I am ready to (belatedly) approve this.