* 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
