Add generic type transpilation

[backfromexile]

This PR adds functionality to the type transpiler to also transpile custom generic types correctly.
For example, the following C# class

[TranspilationSource]
public class Message<T> {
    public Guid Id { get; set; }
    public T Content { get; set; }
}

would be transpiled to this TypeScript type

export type Message<T> = {
    id: string;
    content: T;
}

This also works for nested generic types as long as the type definition is either marked as [TranspilationSource] or the type is known as one of the generic types with built-in support (e.g. certain collection and dictionary types, Nullable<T>).

Transpilation also works with concrete generic type arguments in inheritance chains.

I'd love to get feedback for this addition/change.

[nenoNaninu]

Thanks for the PR! Perhaps the use of ConstructUnboundGenericType() is due to the implementation within SourceTypeMapper.MapTo(), but I think ConstructedFrom is appropriate.

[nenoNaninu]

Suggested change

	if (SymbolEqualityComparer.Default.Equals(typeSymbol.GetUnboundedType(), Assign))
	var symbol = (typeSymbol as INamedTypeSymbol)?.ConstructedFrom ?? typeSymbol;
	if (SymbolEqualityComparer.Default.Equals(symbol, Assign))

[backfromexile]

This won't work if we want to support custom type mappers (#165) for concrete generic types (e.g. a custom mapper/translator that works for Something<string> but not for Something<T>), so this will require a change for the implementation of #165.

[backfromexile]

This is non-blocking for this PR though imo, just something we need to consider if we want to support custom type mappers in the future.

[backfromexile]

Thanks for the PR! Perhaps the use of ConstructUnboundGenericType() is due to the implementation within SourceTypeMapper.MapTo(), but I think ConstructedFrom is appropriate.

I'll have a look, I personally found it easier to wrap my head around unbounded types than having generic and concrete generic types, but I'll change it if it's really just using ConstructedFrom instead.
I only have little experience with the compilation APIs so I appreciate your feedback!

[nenoNaninu]

Sorry for the delay in the review. LGTM!