Add typescript declaration for index.js

[JohnnyStreet]

Is your feature request related to a problem? Please describe.
I have to build a typescript declaration file and include that in my routine every time I use the library (slightly annoying).

Describe the solution you'd like
Add a typescript declaration to the package by doing the following:
npx tsc --declaration --emitDeclarationOnly --allowJs .\index.js

Describe alternatives you've considered
I can do this myself each time but it's nice to see it included so a simple npm install gets people where they need to be.