Add TypeScript declaration files

[danielrearden]

The DefinitelyTyped definitions are pretty misaligned with the Flow definitions. A good example is ConnectionConfig.nodeType which is typed as GraphQLObjectType in Flow (here) but is effectively typed as GraphQLOutputType in DT (here). I'll do a separate PR to sync these with Flow so that there's a clear record of what changed. The only thing I did sync was the comments to reduce the diff for the next PR.

[codecov]

Codecov Report

Merging #279 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #279   +/-   ##
=======================================
  Coverage   99.49%   99.49%           
=======================================
  Files          19       19           
  Lines         595      595           
  Branches       38       38           
=======================================
  Hits          592      592           
  Misses          3        3           

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