feat(macro): improve argument parsing (#2085)

* Improve macro argument parsing

* Make \above a primitive command

* Fix screenshotter data

* Normalize argument where necessary

* Improve argument location info

* Update comments

* Minor refactor

* Modularize group parsers

* Allow braced and blank size argument

for non-strict mode and \genfrac, respectively.

* Minor refactor & update comments

* Remove raw option in parseStringGroup

* Update tests

* Fix { delimited parameter

* Update tests

* Update tests

* Normalize argument in \genfrac

* Update tests

* Consume space before scanning an optional argument

* Fix \\, \newline, and \cr behavior

* Fix flow error

* Update comments

* Remove unnecessary mode switching

Parser mode affects neither fetch nor consume.

* Allow single (active) character macro

* Add function property `primitive`

* Set \mathchoice and \*style primitive

* Separate size-related improvements out to #2139

* Fix flow error

* Update screenshots

* Update demo example

* Add a migration guide

* Fix capitalization

* Make a primitive function unexpandable

* Update screenshots

* Update screenshots

* Revert "Document \def doesn't support delimiters (#2288) (#2289)"

This reverts commit f96fba6f7f.

* Update comments, errors, and tests

* Update screenshots

src/defineFunction.js

@@ -83,6 +83,9 @@ export type FunctionPropSpec = {
 
     // Must be true if the function is an infix operator.
     infix?: boolean,
+
+    // Whether or not the function is a TeX primitive.
+    primitive?: boolean,
 };
 
 type FunctionDefSpec<NODETYPE: NodeType> = {|
@@ -128,6 +131,7 @@ export type FunctionSpec<NODETYPE: NodeType> = {|
     allowedInMath: boolean,
     numOptionalArgs: number,
     infix: boolean,
+    primitive: boolean,
 
     // FLOW TYPE NOTES: Doing either one of the following two
     //
@@ -186,6 +190,7 @@ export default function defineFunction<NODETYPE: NodeType>({
             : props.allowedInMath,
         numOptionalArgs: props.numOptionalArgs || 0,
         infix: !!props.infix,
+        primitive: !!props.primitive,
         handler: handler,
     };
     for (let i = 0; i < names.length; ++i) {
@@ -223,6 +228,10 @@ export function defineFunctionBuilders<NODETYPE: NodeType>({
     });
 }
 
+export const normalizeArgument = function(arg: AnyParseNode): AnyParseNode {
+    return arg.type === "ordgroup" && arg.body.length === 1 ? arg.body[0] : arg;
+};
+
 // Since the corresponding buildHTML/buildMathML function expects a
 // list of elements, we normalize for different kinds of arguments
 export const ordargument = function(arg: AnyParseNode): AnyParseNode[] {