Add mhchem extension (#1436)

* A proposal for support of mhchem

* Create extension

* Add index

* Remove arrow function

* Fix lint errors in macros.js

* Remove mhchem from build

* Move code from macros.js to mhchem.js

* Add minified mhchem

* Remove arrow head mods. Disable eslint.

* Fix space handling

* Update code to mhchem v3.3.0

* Tweak arrows to reduce diff

* Raise \tripledash

* Edit docs and webpack

* Update package.json

* Remove wrap. Remove manual.

* Correct second brush stroke in \xrightleftarrows

* Added patch file

* Add import line

* Add mhchem to eslint ignore

docs/support_table.md

@@ -206,7 +206,7 @@ table td {
 |\cdot|$\cdot$||
 |\cdotp|$\cdotp$||
 |\cdots|$\cdots$||
-|\ce |<span style="color:firebrick;">Not supported</span>|A PR is pending
+|\ce |${\mathrm{C}{\vphantom{X}}_{\smash[t]{6}}\mathrm{H}{\vphantom{X}}_{\smash[t]{5}}{-}\mathrm{CHO}}$|`\ce{C6H5-CHO}` Requires an [extension](https://github.com/KaTeX/KaTeX/tree/master/contrib/mhchem/)|
 |\cee|<span style="color:firebrick;">Not supported</span>|Deprecated by mhchem
 |\centerdot|$a\centerdot b$|`a\centerdot b`|
 |\cf|<span style="color:firebrick;">Not supported</span>|Deprecated by mhchem|
@@ -813,7 +813,7 @@ table td {
 |\providecommand|$\providecommand\greet{\text{Hello}} \greet$|`\providecommand\greet{\text{Hello}} \greet`|
 |\psi|$\psi$||
 |\Psi|$\Psi$||
-|\pu||A PR is pending|
+|\pu |${123~\mathchoice{\textstyle\frac{\mathrm{kJ}}{\mathrm{mol}}}{\frac{\mathrm{kJ}}{\mathrm{mol}}}{\frac{\mathrm{kJ}}{\mathrm{mol}}}{\frac{\mathrm{kJ}}{\mathrm{mol}}}}$|`\pu{123 kJ//mol}` Requires an [extension](https://github.com/KaTeX/KaTeX/tree/master/contrib/mhchem/)|
 
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