Add typescript compatibility (.d.ts)

Importing this module in a typescript project fails.
```
Could not find a declaration file for module 'react-flags'. '/project/node_modules/react-flags/lib/react-flags.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/react-flags` if it exists or add a new declaration (.d.ts) file containing `declare module 'react-flags';`  TS7016
```