Replace the deprecated esprima-fb parser with flow-parser, on the RN website

Summary:
(I changed a ton from when I previously submitted this PR so please take another look if you already did.)

PROBLEM: the no-longer-maintained `esprima-fb` parser does not support class properties, leading our website docgen to die if we use class properties, which we're gonna do real soon now
SOLUTION: use `flow-parser` instead, which the flow team is maintaining including all the fancy-pants ES? stuff that FB uses internally.

This removes the `esprima-fb` parser from jsdocs and replaces it with `flow-parser`. It's almost the same, I checked by diffing all the parser json output and it only had a few irrelevant differences. I had to add a file of constants so that we could remove esprima-fb altogether, too.

This also adds a couple unit tests, so that we can test that jsDocs works programmatically. They don't run if you run the regular RN tests, you have to run `npm test` from the `/website/` subdirectory.
Closes https://github.com/facebook/react-native/pull/9890

Differential Revision: D3865629

Pulled By: bestander

fbshipit-source-id: 8f561b78ca4a02f3f7b45e55904ec2fa911e3bb6