- Add lib or ref assemblies for the netstandard2.0 target framework

Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder do not have exact matches in the other location. Consult the list of actions below:

`Exception` is a reserved exception type

This compiler extension should not be implemented in an assembly containing a reference to Microsoft.CodeAnalysis.Workspaces. The Microsoft.CodeAnalysis.Workspaces assembly is not provided during command line compilation scenarios, so references to it could cause the compiler extension to behave unpredictably. (https://github.com/dotnet/roslyn-analyzers/blob/main/docs/rules/RS1038.md)

Argument of type 'ImmutableDictionary<string, string>' cannot be used for parameter 'properties' of type 'ImmutableDictionary<string, string?>' in 'Diagnostic Diagnostic.Create(DiagnosticDescriptor descriptor, Location? location, ImmutableDictionary<string, string?>? properties, params object?[]? messageArgs)' due to differences in the nullability of reference types.

Argument of type 'ImmutableDictionary<string, string>' cannot be used for parameter 'properties' of type 'ImmutableDictionary<string, string?>' in 'Diagnostic Diagnostic.Create(DiagnosticDescriptor descriptor, Location? location, ImmutableDictionary<string, string?>? properties, params object?[]? messageArgs)' due to differences in the nullability of reference types.

Dereference of a possibly null reference.

Argument of type 'ImmutableDictionary<string, string>' cannot be used for parameter 'properties' of type 'ImmutableDictionary<string, string?>' in 'Diagnostic Diagnostic.Create(DiagnosticDescriptor descriptor, Location? location, ImmutableDictionary<string, string?>? properties, params object?[]? messageArgs)' due to differences in the nullability of reference types.

Dereference of a possibly null reference.