Bump Microsoft.NET.Test.Sdk from 17.5.0 to 17.8.0

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 17.5.0 to 17.8.0.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Changelog](https://github.com/microsoft/vstest/blob/main/docs/releases.md)
- [Commits](microsoft/vstest@v17.5.0...v17.8.0)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

+Mᐁ includes

Remove version badge from package readme

Since the badge is generic and points to the latest version, it would be 
confusing when navigating a specific version on nuget.org to see a 
badge for a different version.

Bump files with dotnet-file sync

# devlooped/oss

- Move changelog config to .github, switch to gem devlooped/oss@b7ce2be
- Make src-level readme visible for easier editing devlooped/oss@bfe0f2a
- Add icon prefix to match dependabot devlooped/oss@2a39a42

Expose AnonymousObserver

In certain scenarios, it might be useful to be able to create the anonymous observer outside the Subscribe overloads.

For example, the official guidance on DiagnosticSource filtering recommends creating an anonymous observer to combine with the predicate to filter events.

There may be other scenarios too where not having to manually create an IObserver<T> could be useful.

Fixes #8

CI build badge is not necessary or useful in package readme

🖉 Update changelog with v1.0.2

📝 Add minimal code example in Usage section

🐛 Fix build error for legacy and nullable subjects

When copying the implementation from System.Reactive, we improperly brought HasObservers as an override, but it should be just virtual, since we don't have a SubjectBase class.

Add readme, sponsors, dogfooding sections