Port katex-a11y.js to KaTeX (#2062)

* copy katex-a11y.js from webapp to contrib/to-a11y-string.js

* first pass of updating to-a11y-string to use flow

* Set up test harness and write some tests
- add flow types for jest
- create wallaby config file
- add prettier as a dep and set up .prettierrc so that we use .toMatchInlineSnapshot() and have jest insert the snapshot for us
- add lots of TODOs (I'll create tickets for these later)

* Add some  to get the tests passing for now

* remove commented out parts of wallaby.js config file

* remove some stale strings, fix flow issue, add more coverage

* add , add few more tests, handle middle

* fix minor nits in tests

* rename to-a11y-string to render-a11y-string, updatte webpack config to build extension

* add test for \boxed

* remove commented out code

webpack.common.js

@@ -45,6 +45,10 @@ const targets /*: Array<Target> */ = [
         name: 'contrib/mathtex-script-type',
         entry: './contrib/mathtex-script-type/mathtex-script-type.js',
     },
+    {
+        name: 'contrib/render-a11y-string',
+        entry: './contrib/render-a11y-string/render-a11y-string.js',
+    },
 ];
 
 /**