Extensions: follow-ups for metadata

[jcouv]

This issue is referenced in source.

The list includes:

• [ ] switch away from ordinals in skeleton type names
• [ ] confirm on parameter ordinal used in EmitExpression.EmitParameterIdExpression
• [ ] consider adding private constructor to extension skeleton types?
• [ ] consider adding ExtensionAttribute to extension skeleton types even when no member method
• [ ] consider optimizing lookup using presence of [Extension] attribute
• [ ] User may apply attribute to set special name. We should honor that on the implementation method
• [ ] Metadata: copying attributes (synthesized attributes like nullability? we copy some attributes from enclosing method to closure type, see previous work that implemented that)
• [ ] Implementation methods should be considered implicitly declared
• [ ] Manual testing with reflection
• [ ] Ping known metadata consumers (SandCastle, decompilers)
• [ ] Test ref-only assemblies (should already work, needs test)
• [ ] Skeleton methods should throw NotSupportedException (currently using throw null)

Relates to test plan https://github.com/dotnet/roslyn/issues/76130