[context_watch] Removes the rxdart dependency

I ran into conflicts with the rxdart dependency across multiple projects. It's very common. Usually, I avoid any package depending on rxdart because of so many breaking changes in the past. This PR entirely removes the rxdart dependency and while making it compatible with even more versions: compatability before: `rxdart: ">=0.27.7 <0.28.0` compatability now: `rxdart: ">=0.26.0 <0.29.0"`  The only thing I had to replace was the usage of `ValueStream`. It was fairly easy using `dynamic` checks. Similar to `tryDispose` in `context_ref`. `rxdart: 0.26.0` is old, but it was easy to add support. Hence I added it right away.