FIX: ArgumentException Ctors support (closes #111)

Thanks @sharwell for reporting this bug.
You can cover `MethodDeclarationSyntax` and `ConstructorDeclarationSyntax` at the same time by simplifying this entire method to the following, which also avoids an exception in some other cases I'll mention as a comment in the issue:

```csharp
return node.FirstAncestorOrSelf<BaseMethodDeclarationSyntax>()?.ParameterList;
```