Update Semantic Search to work when ServiceHub is disabled

[sharwell]

• Fix some design time build warnings
• Retarget Semantic Search reference assemblies from net8.0 to netstandard2.0
• Support Semantic Search in devenv.exe
• Add Semantic Search to the preview features list

Fixes #72617

[tmat]

We should not limit semantic search to netstandard2.0 when running on ,NET 8. That unnecessarily limits the APIs and language features the user can use.

[sharwell]

That unnecessarily limits the APIs and language features the user can use.

In theory, yes. In practice, it doesn't seem likely to make a difference. The approach used in this pull request also matches the behavior the compiler expects for interacting with certain other public APIs, e.g. source generator implementations, so this restriction is not without precedent.

The alternative is to generate and ship multiple sets of reference assemblies, adding a significant amount of complexity to the build and making it unpredictable which set of APIs will be usable for any given search.

It seems like this is something we can add later if there is a demonstrated need, particularly considering that this future addition would not remove any APIs being provided by netstandard2.0.