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

.prettierrc

@@ -0,0 +1,7 @@
+{
+    "trailingComma": "all",
+    "tabWidth": 4,
+    "semi": true,
+    "singleQuote": false,
+    "bracketSpacing": false
+}