Add support for \expandafter, \noexpand, \edef, \let, and \long (#2122)

* Add support for \expandafter

* Add support for \noexpand

* Add support for \edef

* Update comments

* Allow \long before macro definition

* Update documentation

* Update comments

* Fix defPrefix

* Add support for \let

* Update documentation

* Print error token

* Update documentation

* Check whether command is expandable

* Add tests

* Fix token order

* Make noexpand a Token property

* Throw error if control sequence is undefined when expanding

* Rename expandableOnly to expandOnly

* Make unexpandable macro property

* Move \expandafter to macros.js

* Add TODO

* Fix merge conflict

* Update a test case

* Remove unused functions in MacroContextInterface

* Update comments

* Refactor code

* Move \noexpand to macros

* Update MacroExpander.js

* Add a test case

* Separate control sequence check to a function

* Add support for \futurelet

* Separate RHS getter to a function

* Update documentation

* Move expandOnly logic to expandOnce

* Refactor code and update comments

Co-authored-by: Kevin Barabash <kevinb@khanacademy.org>

docs/supported.md

@@ -297,6 +297,10 @@ Direct Input: $∀ ∴ ∁ ∵ ∃ ∣ ∈ ∉ ∋ ⊂ ⊃ ∧ ∨ ↦ → ←
 |:-------------------------------------|:------
 |$\def\foo{x^2} \foo + \foo$           | `\def\foo{x^2} \foo + \foo`
 |$\gdef\bar#1{#1^2} \bar{y} + \bar{y}$ | `\gdef\bar#1{#1^2} \bar{y} + \bar{y}`
+|                                      | `\edef\macroname#1#2…{definition to be expanded}`
+|                                      | `\xdef\macroname#1#2…{definition to be expanded}`
+|                                      | `\let\foo=\bar`
+|                                      | `\futurelet\foo\bar x`
 |                                      | `\global\def\macroname#1#2…{definition}`
 |                                      | `\newcommand\macroname[numargs]{definition}`
 |                                      | `\renewcommand\macroname[numargs]{definition}`
@@ -306,11 +310,13 @@ Macros can also be defined in the KaTeX [rendering options](options.md).
 
 Macros accept up to nine arguments: #1, #2, etc.
 
-`\gdef` and `\global\def` macros will persist between math expressions.
+`\gdef`, `\xdef`, `\global\def`, `\global\edef`, `\global\let`, and `\global\futurelet` will persist between math expressions.
+
+KaTeX has no `\par`, so all macros are long by default and `\long` will be ignored.
 
 Available functions include:
 
-`\char` `\mathchoice` `\TextOrMath` `\@ifstar` `\@ifnextchar` `\@firstoftwo` `\@secondoftwo` `\relax`
+`\char` `\mathchoice` `\TextOrMath` `\@ifstar` `\@ifnextchar` `\@firstoftwo` `\@secondoftwo` `\relax` `\expandafter` `\noexpand`
 
 @ is a valid character for commands, as if `\makeatletter` were in effect.