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

wallaby.js

@@ -0,0 +1,39 @@
+const babelConfig = require("./babel.config.js");
+
+module.exports = function(wallaby) {
+    const tests = [
+        "test/*-spec.js",
+        "contrib/**/test/*-spec.js",
+    ];
+
+    return {
+        tests,
+
+        // Wallaby needs to know about all files that may be loaded because
+        // of running a test.
+        files: [
+            "src/**/*.js",
+            "test/**/*.js",
+            "contrib/**/*.js",
+            "submodules/**/*.js",
+            "katex.js",
+
+            // These paths are excluded.
+            ...tests.map((test) => `!${test}`),
+        ],
+
+        // Wallaby does its own compilation of .js files to support its
+        // advanced logging features.  Wallaby can't parse the flow and
+        // JSX in our source files so need to provide a babel configuration.
+        compilers: {
+            "**/*.js": wallaby.compilers.babel(babelConfig),
+        },
+
+        env: {
+            type: "node",
+            runner: "node",
+        },
+
+        testFramework: "jest",
+    };
+};