Adds typescript types

[ajmath]

If you're interested in adding some validation for the types, I can help set that up but it would require adding react, react-native, and typescript as dev dependencies which I didn't want to do since there are none currently. Basically, add those dependencies and then add an npm script with npx tsc --noEmit ./index.d.ts.

[kyaroru]

hi @ajmath thanks for the suggestion.. but I not so understand on how it works 😢 (no TS experience)

[ajmath]

That's all fine, it's mergeable as is now. Just something to consider for the future if you start adding some CI.

[Josephdias92]

Much needed PR, otherwise I have to jump to GitHub readme everytime to see the type of value a property expects

[jdnichollsc]

@kyaroru do you want to merge this?