Provide OSGI support

JSpecify annotations could also be used in OSGI environments if `MANIFEST.MF` would have bundle entries like e.g.
```
Bundle-ManifestVersion: 2
Bundle-Name: JSpecify annotations
Bundle-SymbolicName: org.jspecify.annotations
Bundle-Version: 0.3.0
Export-Package: org.jspecify.annotations
[...]
```
Please provide a solution and JSpecify annotations could be used out of the box.