Merge TypeScript declaration with DefinitelyTyped

The current [TypeScript declaration file](https://github.com/itinance/react-native-fs/blob/master/index.d.ts) is missing a few functions (e.g. `appendFile`) that are covered by the [one in DefinitelyTyped's repo](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react-native-fs/index.d.ts) (@pocesar and @josephroquedev are listed as the maintainers there). The one on DefinitelyTyped is also more complete, avoiding the `any` type for some options (e.g. encodings).

Would there be any objections to merging the two declaration files here?