export typescript types in package.json

[RikSchefferAmsterdam]

When adding this import to a react native project import {ReactNativePlugin} from '@microsoft/applicationinsights-react-native', I get the following error:

Could not find a declaration file for module '@microsoft/applicationinsights-react-native'. '/Users/rik/workspace/aapp_app_mobile/node_modules/@microsoft/applicationinsights-react-native/dist-esm/index.js' implicitly has an 'any' type.
  There are types at '/Users/rik/workspace/aapp_app_mobile/node_modules/@microsoft/applicationinsights-react-native/types/index.d.ts', but this result could not be resolved when respecting package.json "exports". The '@microsoft/applicationinsights-react-native' library may need to update its package.json or typings.ts(7016)

This can easily be solved be changes the package.json exports. So therefore I made this pull requests.

[siyuniu-ms]

https://webpack.js.org/guides/package-exports/#conditions
based on this doc, this syntax is correct.
Test via sample app, works good locally.