Move SK.CoreSkills to new package: SK.Skills.Core (#1623)

### Motivation and Context
Slimming down the kernel core project, nuget package.

### Description
These "Core Skills" are good ones to have, but are not fundamental to
the operation of SK. They will continue to be included in the
"Microsoft.SemanticKernel" nuget metapackage, but if a developer chooses
to just install the minimal "Microsoft.SemanticKernel.Core", they may
not want these skills. This packages them so that they are now optional.

### Contribution Checklist
<!-- Before submitting this PR, please make sure: -->
- [X] The code builds clean without any errors or warnings
- [X] The PR follows SK Contribution Guidelines
(https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
- [X] The code follows the .NET coding conventions
(https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions)
verified with `dotnet format`
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

---------

Co-authored-by: name <email>
Co-authored-by: Lee Miller <[email protected]>

dotnet/samples/KernelSyntaxExamples/KernelSyntaxExamples.csproj

@@ -35,6 +35,7 @@
     <ProjectReference Include="..\..\src\Extensions\Planning.ActionPlanner\Planning.ActionPlanner.csproj" />
     <ProjectReference Include="..\..\src\Extensions\Planning.SequentialPlanner\Planning.SequentialPlanner.csproj" />
     <ProjectReference Include="..\..\src\Connectors\Connectors.Memory.Pinecone\Connectors.Memory.Pinecone.csproj" />
+    <ProjectReference Include="..\..\src\Skills\Skills.Core\Skills.Core.csproj" />
     <ProjectReference Include="..\..\src\Skills\Skills.OpenAPI\Skills.OpenAPI.csproj" />
     <ProjectReference Include="..\..\src\Skills\Skills.Grpc\Skills.Grpc.csproj" />
     <ProjectReference Include="..\..\src\Connectors\Connectors.Memory.Qdrant\Connectors.Memory.Qdrant.csproj" />