Add type declaration file

[vreynolds]

Is your feature request related to a problem? Please describe.
Related to #88
TypeScript users are missing out on the benefits of a typed API for the library, because we don't have types.

Describe the solution you'd like
Add a type declaration file.

This is the first step in the TS port project, which will allow us to understand the library API and provide user value faster, while we continue on porting the rest of the library to TS.

Describe alternatives you've considered
Wait until we complete the TS port. In the long run, it's more maintainable to have the source code written in TS instead of the standalone types file. We intend to replace the standalone types file soon, so the tradeoff seems worthwhile in this case.

Additional context