Support for top-level \newline and \\ in inline math (#1298)

* Support for top-level \newline and \\ in inline math

This was a little tricky because `\\` was defined as an endOfExpression.
Instead made `\\` a termination specific to an array environment.
Outside an array environment, buildHTML handles the `cr` object,
resulting in a `.newline` class.  Currently this turns into a
`display: block` (with appropriate vertical spacing) only in inline math,
matching LaTeX.

* Simplify code

* Fix Jest errors

* NewLine screenshot test

* Bug fix: \\ only works at top level of inline

* Add \newline and \cr to test

* Switch test to pmatrix

* Add vertical space test

* Add \\ vs. \newline tests

* Fix flow errors

* Add \cr test

* Add documentation for \\ at top level

* Comment out newRow

* Fix commenting out

src/environments/array.js

@@ -64,7 +64,7 @@ function parseArray(
     numHLinesBeforeRow.push(getNumHLines(parser));
 
     while (true) {  // eslint-disable-line no-constant-condition
-        let cell = parser.parseExpression(false, undefined);
+        let cell = parser.parseExpression(false, "\\\\");
         cell = new ParseNode("ordgroup", cell, parser.mode);
         if (style) {
             cell = new ParseNode("styling", {
@@ -100,7 +100,7 @@ function parseArray(
             row = [];
             body.push(row);
         } else {
-            throw new ParseError("Expected & or \\\\ or \\end",
+            throw new ParseError("Expected & or \\\\ or \\cr or \\end",
                                  parser.nextToken);
         }
     }