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

webpack.common.js

@@ -25,6 +25,10 @@ const targets /*: Array<Target> */ = [
         entry: './contrib/auto-render/auto-render.js',
         library: 'renderMathInElement',
     },
+    {
+        name: 'contrib/mhchem',
+        entry: './contrib/mhchem/mhchem.js',
+    },
     {
         name: 'contrib/copy-tex',
         entry: './contrib/copy-tex/copy-tex.webpack.js',