Improve react-native link

[mitsuhiko]

This heavily improves react-native link and unlink.

The idea is that we do some stuff on link that generally makes sense and then persist through unlink and relink. This way users can "upgrade" by unlinking and re-linking to newer react-native sentry versions.

[mitsuhiko]

TODO:

• if we launch this after our swift -> objc rewrite is through we should remove the swift flag flipping
• consider undoing old SENTRY_ORG exports and others in the react native build script we let users patch in the past. If we don't do that we should add some upgrade docs so that users can do that manually.

[mitsuhiko]

All todo steps are done now and the docs are also updated. We should be careful when pulling this in as the docs will rebuild automatically. So this should only be merged when we do the actual release.