Microsoft.CodeAnalysis.Semantics renamed to Microsoft.CodeAnalysis.Operations

Addresses Preview and Documentation items from #22719

1. Rename `ILocalInitializer` to `IVariableInitializer`
2. Refactor OperationFactory methods
3. Change type of `IVariableDeclaration.Initializer` from `IOperation` to `IVariableInitializer`

Refactored IVariableDeclaration to IVariableDeclarationGroup, which allows for multiple declared symbols as part of a single declaration.

The TestUtilities project should be our portable unit test helper.  This is part one of rearranging our sources to reflect that reality.  I'm separating it out because it involves a significant code move that
will make the actual rework part harder to review.

Unit tests are deployment projects and hence cannot safely reference each other.