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

.eslintrc

@@ -75,7 +75,8 @@
     "env": {
         "es6": true,
         "node": true,
-        "browser": true
+        "browser": true,
+        "jest": true
     },
     "settings": {
         "react": {