Add Typescript Definitions

[nataly87s]

following issue #12

[codecov]

Codecov Report

Merging #19 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #19   +/-   ##
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  Coverage     100%   100%           
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  Files           4      4           
  Lines          53     53           
  Branches       12     12           
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  Hits           53     53

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[philipp-spiess]

This looks great! Thank you so much!

I'd like to also add some tests for the typescript declarations, much like in the Immutable.js repo. Do you want to take a look here as well? No pressure though 🙂

I think we also need to make some changes to the build system so that the type definitions are properly exposed - I'm not sure what's the best way to do this though. I'd again take a look at the Immutable.js repo 🙈