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

package.json

@@ -56,6 +56,7 @@
     "mkdirp": "^0.5.1",
     "pako": "^1.0.8",
     "postcss-loader": "^3.0.0",
+    "prettier": "^1.18.2",
     "query-string": "^6.2.0",
     "rimraf": "^2.6.3",
     "rollup": "^1.2.2",