feat: Support \phase (#2406)

* Support \phase

* Update screenshots

* Update documentation

* Add a11y test

* Edit MathML notation to phasorangle

* Move code to enclose.js

Co-authored-by: Kevin Barabash <kevinb@khanacademy.org>

test/katex-spec.js

@@ -2479,6 +2479,15 @@ describe("A actuarial angle builder", function() {
     });
 });
 
+describe("\\phase", function() {
+    it("should fail in text mode", function() {
+        expect`\text{\phase{-78.2^\circ}}`.not.toParse();
+    });
+    it("should not fail in math mode", function() {
+        expect`\phase{-78.2^\circ}`.toBuild();
+    });
+});
+
 describe("A phantom parser", function() {
     it("should not fail", function() {
         expect`\phantom{x}`.toParse();

test/screenshotter/ss_data.yaml

@@ -318,6 +318,7 @@ Phantom: \begin{array}{l}
             \text{a \phantom{123}} b \hphantom{\frac{1}{2}}=c \vphantom{101112} d \\
             \sqrt{\mathstrut a} + \sqrt{\mathstrut d}
          \end{array}
+Phase: 120\text{V}\phase{-78.2^\circ}\;\Large\phase{78.2^\circ}
 Pmb: \mu\pmb{\mu}\pmb{=}\mu\pmb{+}\mu
 PrimeSpacing: f'+f_2'+f^{f'}
 PrimeSuper: x'^2+x'''^2+x'^2_3+x_3'^2