Move \global and \def to functions (#2138)

* Add parseNode type `internal`

* Move \def and \newcommand to functions

* Fix Flow error

* Separate \global

Move \newcommand back to macros for now

* Rename assignment.js to def.js

* Update test cases

* Add comments

* Update a test case

contrib/render-a11y-string/render-a11y-string.js

@@ -655,6 +655,11 @@ const handleObject = (
             break;
         }
 
+        case "internal": {
+            // internal nodes are never included in the parse tree
+            break;
+        }
+
         case "html": {
             buildA11yStrings(tree.body, a11yStrings, atomType);
             break;