Move JSX types to local namespace

[jcfranco]

Enhancement

It'd be nice to move the global JSX typings to a local namespace since TypeScript 2.8 now resolves JSX via the factory function. This is also recommended in their release notes:

Going forward, we recommend that new JSX-oriented libraries avoid placing JSX in the global scope, and instead export it from the same location as the respective factory function.

[matt-gadd]

We plan to update this when we move to TypeScript 2.9