Fix the TypeScript definition for TypeScript 3.5

[mskec]

Similar to #38

The current definition doesn't allow accessing nested properties.

const obj = { a: { a1: '1' } };
const ccObj = camelcaseKeys(obj, { deep: true });
console.log(ccObj.a.a1); // 'a1' does not exist on type 'unknown'.

[sindresorhus]

The current behavior is intentional. #38 was about input arguments. This is about return value. The correct solution is to make it generic so it preserves the input type.

[mskec]

@sindresorhus what do you suggest how to make it generic? I'm quite new with typescript

[sindresorhus]

I don't have the time to look into this now. Moving this to #50.