Fix MA0048 in case when first type is not first node

[FrediKats]

Option "only_validate_first_type" now works more like "only_validate_first_node".

foreach (var node in root.DescendantNodesAndSelf(descendIntoChildren: node => !IsTypeDeclaration(node)))

This block of code returns CompilationUnit and if (node.SpanStart < symbolNode.SpanStart) always works as if (0 < symbolNode.SpanStart).

PR contains:

• test to confirm this problem (namespace before type).
• fix with comparing only with other type declarations (skip nodes that is not type declaration)

[meziantou]

Thanks for your contribution 🚀