Add flow types for all public APIs

[davidtheclark]

I create a Flow definition file of this library for Mapbox Studio, and chatted a little with @lucaswoj about putting that definition in this repo.

I can think of some reasons that it would be a good idea to maintain the definitions in this repo:

• Closer to the source code
• Updates would always go out with new releases
• If we ever figure out a way to autogenerate the definition from the thorough JSDoc comments it can happen here

I can't think of any reason not to put the definitions here. Can anybody else?

And if we do put them here, where should they go? I'm happy to open a PR.

[lucaswoj]

The flow definitions can go in this repository's root directory. PR away!

[davidtheclark]

Are all the properties exposed in https://github.com/mapbox/mapbox-gl-js/blob/master/js/mapbox-gl.js intended to be part of the public API?

If so, we should document version, util, and config, and add them all the Flow definition.

(I was thinking about this because Studio uses mapboxgl.config so needed it in the definition.)

[lucaswoj]

• 👍 version I added this property and failed to document it. This can and should be documented.
• 👎 util There has been a lot of debate about whether or not we should document this. The current consensus is that we should not. Unexport or document mapboxgl.util #1408
• ❓ config I'm not aware of any existing discussion about documenting this property. My opinion is that we should look into simplifying the architecture and then document it. A simpler architecture would be dropping the config module in favor of top level properties on mapboxgl.

[lucaswoj]

We now have flow built in! Reframing this ticket to be about adding flow types to all public APIs.