Make ParseNode value payload and defineFunction handler functions type-safe (#1276)

* Make ParseNode `value` payload type-safe.

* Make defineFunction handlers aware of ParseNode data types.

* Add `type` to all function definitions to help determine handler return type.

* Added unit test for case caught only in screenshot test and fixed issue.

* Rename some symbol `Group`s to avoid conflicts with `ParseNode` groups.

Symbol `Group`s are also used as `ParseNode` types. However, `ParseNode`s of
these types always contain a raw text token as opposed to any structured
content. These `ParseNode`s are passed as arguments into function handlers to
create more semantical `ParseNode`s with more structure.

Before this change, "accent" and "op" were both symbol `Group`s and `ParseNode`
types. With this change, these two types (the raw accent token `ParseNode`, and
the structured semantical `ParseNode` are separated for better type-safety on
the `ParseNode` payload).

* stretchy: Remove FlowFixMe for a forced typecast that's no longer needed.

test/katex-spec.js

@@ -407,6 +407,7 @@ describe("A subscript and superscript parser", function() {
 
     it("should not fail when there is no nucleus", function() {
         expect("^3").toParse();
+        expect("^3+").toParse();
         expect("_2").toParse();
         expect("^3_2").toParse();
         expect("_2^3").toParse();