feat: Support {CD} (#2396)

* Support {CD}

* Edit screenshotter test to fit on one page

* Update screenshots

* Remove bogus Safari screenshot

* Edit documentation to avoid tag conflicts and explain delimiters

* Add type annotations

* Add bogus safari screenshot

* Update with real Safari screenshot

* Set label vertical alignment

* Revise call to parseExpression() per PR 2085 changes to macro parsing

* Update Firefox screenshot

* Pick up review comments

* Add unit tests and snapshot.

* Tighten up label collection.

* Better loop index

* remove extra space

* Picked up comments. Added a parse check. Added a test.

Co-authored-by: ylemkimon <y@ylem.kim>
Co-authored-by: Kevin Barabash <kevinb@khanacademy.org>
Co-authored-by: Kevin Barabash <kevinb7@gmail.com>

test/screenshotter/ss_data.yaml

@@ -86,6 +86,9 @@ Cases: |
         a &\text{if } b \\
         c &\text{if } d
     \end{rcases}⇒…
+CD:
+  tex: \begin{CD} A @<a<< B @>>b> C \\ @| @AcAA @VVdV \\ D @= E @>>> F \end{CD}
+  display: 1
 Colors:
     tex: \blue{a}\textcolor{#0f0}{b}\textcolor{red}{c}
     nolatex: different syntax and different scope