Support Reaction Arrows (#1078)

* Support Reaction Arrows

This PR is written to supply reaction arrows for a future `mhchem` extension. `mhchem` uses seven reaction arrows. Four of them correspond to extensible arrows already available in KaTeX. This PR creates the other three.

These arrows will also be useful to chemists writing about reactions when `mhchem` is unavailable.

Three new extensible arrows are introduced: `\xrightleftarrows`, `\xrightequilibrium`, and `\xleftequilibrium`.

These names are not `mhchem`’s user-facing function names. In `mhchem`, users would call these arrows with syntax such as: `\ce{A<-->B}`, or `\ce{A<=>>B}`, or `\ce{A<<=>B}`. I’ve provided names that look more like the other extensible arrow names. That’s probably worth some discussion.

* generate screenshots for ReactionArrows

* Increase overlap in arrow shaft

To prevent a small gap when rendering in very large font sizes.

* Adjust upper text vert alignment. Add warning.

* Limit alignment adjustment to \xleftequilibrium

* generate screenshots for reaction arrows

test/screenshotter/ss_data.yaml

@@ -223,6 +223,11 @@ Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x} \left(\vphanto
 PrimeSpacing: f'+f_2'+f^{f'}
 PrimeSuper: x'^2+x'''^2+x'^2_3+x_3'^2
 Raisebox: \frac{a}{a\raisebox{0.5em}{b}} \cdot \frac{a\raisebox{-0.5em}{b}}{a} \cdot \sqrt{a\raisebox{0.5em}{b}} \cdot \sqrt{a\raisebox{-0.5em}{b}} \cdot \sqrt{a\raisebox{0.5em}{b}\raisebox{-0.5em}{b}}
+ReactionArrows: |
+    \begin{matrix}
+        A \xrightleftarrows{} B \xrightequilibrium{} C \xleftequilibrium{} D \\
+        A \xrightleftarrows{over} B \xrightequilibrium{over} C \xleftequilibrium{over} D
+    \end{matrix}
 RelativeUnits: |
     \begin{array}{ll}
         a\kern1emb^{a\kern1emb^{a\kern1emb}} &