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

src/functions/mclass.js

@@ -71,7 +71,7 @@ defineFunction({
         return {
             type: "mclass",
             mode: parser.mode,
-            mclass: "m" + funcName.substr(5),
+            mclass: "m" + funcName.substr(5), // TODO(kevinb): don't prefix with 'm'
             body: ordargument(body),
             isCharacterBox: utils.isCharacterBox(body),
         };