\color affects following \right, put array cells in their own groups (#1845)

* \color affects following \right

Fix #1844 by giving `leftright` nodes a `rightColor` attribute for how
to color the right bracket.  Now `\color` sets the macro
`\current@color` in the current environment (in particular, resetting
after `\right`), just like `color.sty` does in LaTeX.  This is used to
specially pass the current color into any following `\right` and then
into the `leftright` parse node.

* Add test

* Put each array cell in its own group/namespace

* Improve cell group isolation, add test and TODO

* Improve comments

test/katex-spec.js

@@ -3123,6 +3123,21 @@ describe("A macro expander", function() {
             .toParseLike`11\sqrt[2]{2}11`;
     });
 
+    it("array cells generate groups", () => {
+        expect`\def\x{1}\begin{matrix}\x&\def\x{2}\x&\x\end{matrix}`
+            .toParseLike`\begin{matrix}1&2&1\end{matrix}`;
+    });
+
+    // TODO: This doesn't yet work; before the environment gets called,
+    // {matrix} gets consumed which means that the \def gets executed, before
+    // we can create a group. :-(  Issue #1989
+    /*
+    it("array cells generate groups", () => {
+        expect`\def\x{1}\begin{matrix}\def\x{2}&\x\end{matrix}`
+            .toParseLike`\begin{matrix}&1\end{matrix}`;
+    });
+    */
+
     it("\\gdef changes settings.macros", () => {
         const macros = {};
         expect`\gdef\foo{1}`.toParse(new Settings({macros}));