Combine ReactJSXElementValidator with main module (#28317)

There are too many layers to the JSX runtime implementation. I think
basically everything should be implemented in a single file, so that's
what I'm going to do.

As a first step, this deletes ReactJSXElementValidator and moves all the
code into ReactJSXElement. I can already see how this will help us
remove more indirections in the future.

Next I'm going to do start moving the `createElement` runtime into this
module as well, since there's a lot of duplicated code.

DiffTrain build for [ec160f3](ec160f3)

Author: acdlite

compiled/facebook-www/JSXDEVRuntime-dev.classic.js

@@ -1 +1 @@
-c820097716c3d9765bf85bf58202a4975d99e450
+ec160f32c28ccab798c73ecccbb36ce121e1640e