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/support_table.md

@@ -347,6 +347,7 @@ use `\ce` instead|
 
 |Symbol/Function |  Rendered   | Source or Comment|
 |:---------------|:------------|:-----------------|
+|\edef|$\def\foo{a}\edef\bar{\foo}\def\foo{}\bar$|`\def\foo{a}\edef\bar{\foo}\def\foo{}\bar`|
 |\ell|$\ell$||
 |\else|<span style="color:firebrick;">Not supported</span>|[Issue #1003](https://github.com/KaTeX/KaTeX/issues/1003)|
 |\em|<span style="color:firebrick;">Not supported</span>||
@@ -380,6 +381,7 @@ use `\ce` instead|
 |\exist|$\exist$||
 |\exists|$\exists$||
 |\exp|$\exp$||
+|\expandafter|||
 
 ## F
 
@@ -396,6 +398,7 @@ use `\ce` instead|
 |\frac|$\frac a b$|`\frac a b`|
 |\frak|$\frak{AaBb}$|`\frak{AaBb}`|
 |\frown|$\frown$||
+|\futurelet|||
 
 ## G
 
@@ -571,7 +574,7 @@ use `\ce` instead|
 |\lesseqqgtr|$\lesseqqgtr$||
 |\lessgtr|$\lessgtr$||
 |\lesssim|$\lesssim$||
-|\let|<span style="color:firebrick;">Not supported</span>|[Issue #1413](https://github.com/KaTeX/KaTeX/issues/1413)|
+|\let|||
 |\lfloor|$\lfloor$||
 |\lg|$\lg$||
 |\lgroup|$\lgroup$||
@@ -595,6 +598,7 @@ use `\ce` instead|
 |\lnot|$\lnot$||
 |\lnsim|$\lnsim$||
 |\log|$\log$||
+|\long|||
 |\Longleftarrow|$\Longleftarrow$||
 |\longleftarrow|$\longleftarrow$||
 |\Longleftrightarrow|$\Longleftrightarrow$||
@@ -718,6 +722,7 @@ use `\ce` instead|
 |\nmid|$\nmid$||
 |\nobreak|||
 |\nobreakspace|$a\nobreakspace b$|`a\nobreakspace b`|
+|\noexpand|||
 |\nolimits|$\lim\nolimits_x$|`\lim\nolimits_x`|
 |\normalfont|<span style="color:firebrick;">Not supported</span>||
 |\normalsize|$\normalsize normalsize$|`\normalsize normalsize`|
@@ -1197,6 +1202,7 @@ use `\ce` instead|
 |Symbol/Function |  Rendered   | Source or Comment|
 |:---------------|:------------|:-----------------|
 |\xcancel|$\xcancel{ABC}$|`\xcancel{ABC}`|
+|\xdef|$\def\foo{a}\xdef\bar{\foo}\def\foo{}\bar$|`\def\foo{a}\xdef\bar{\foo}\def\foo{}\bar`|
 |\Xi|$\Xi$||
 |\xi|$\xi$||
 |\xhookleftarrow|$\xhookleftarrow{abc}$|`\xhookleftarrow{abc}`|