Add missing scrollBy TypeScript definition

[danielholmes]

As outlined in the README

Is it a bugfix ?

• Yes, issue scrollBy method definition is missing on Swiper #930

Is it a new feature ?

• No

Describe what you've done:

Added a TypeScript definition for scrollBy to index.d.ts

How to test it ?

import Swiper from 'react-native-swiper'

const swiper: Swiper = new Swiper({})
if (swiper) {
  swiper.scrollBy(1, false)
}

This will now pass the TypeScript compiler whereas before it wouldn't