Exporting more types & interfaces

[SaFrMo]

I'd like to use the PhaseMap type in my project but am running into this error when trying to import: Module '"boardgame.io"' declares 'PhaseMap' locally, but it is not exported.ts(2459)

Is there some other way I should be doing this? I'm looking to define the phases property for a Game object and it looks like PhaseMap should be the way to go, but I can switch to something else if needed.

I looked in the types file and found a couple more that weren't exported, so I put together a quick branch here to resolve. Happy to submit a PR if this is a useful addition. Thanks for the excellent work!

[delucis]

@SaFrMo Sure! A PR for this would be good.

[delucis]

Closed in #927