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/functions/cr.js

@@ -0,0 +1,57 @@
+//@flow
+// Row breaks within tabular environments, and line breaks at top level
+
+import defineFunction from "../defineFunction";
+import buildCommon from "../buildCommon";
+import mathMLTree from "../mathMLTree";
+import { calculateSize } from "../units";
+import ParseError from "../ParseError";
+
+defineFunction({
+    type: "cr",
+    names: ["\\\\", "\\cr", "\\newline"],
+    props: {
+        numArgs: 0,
+        numOptionalArgs: 1,
+        argTypes: ["size"],
+        allowedInText: true,
+    },
+
+    handler: (context, args, optArgs) => {
+        return {
+            type: "cr",
+            // \\ and \cr both end the row in a tabular environment
+            // This flag isn't currently needed by environments/array.js
+            //newRow: context.funcName !== "\\newline",
+            // \\ and \newline both end the line in an inline math environment
+            newLine: context.funcName !== "\\cr",
+            size: optArgs[0],
+        };
+    },
+
+    // The following builders are called only at the top level,
+    // not within tabular environments.
+
+    htmlBuilder: (group, options) => {
+        if (!group.value.newLine) {
+            throw new ParseError(
+                "\\cr valid only within a tabular environment");
+        }
+        const span = buildCommon.makeSpan(["mspace", "newline"], [], options);
+        if (group.value.size) {
+            span.style.marginTop =
+                calculateSize(group.value.size.value, options) + "em";
+        }
+        return span;
+    },
+
+    mathmlBuilder: (group, options) => {
+        const node = new mathMLTree.MathNode("mspace");
+        node.setAttribute("linebreak", "newline");
+        if (group.value.size) {
+            node.setAttribute("height",
+                calculateSize(group.value.size.value, options) + "em");
+        }
+        return node;
+    },
+});