feat: enhance nested type handling with custom converters for serialization and deserialization

[glucaci]

This pull request enhances the source generator logic in Yamlify.SourceGenerator to better support custom converters for nested types during YAML serialization and deserialization. The main improvement is that, when a nested type has a custom converter, the generated code now wires up the generated read and write methods to the custom converter, ensuring correct handling of nested serialization logic.

Custom Converter Integration for Nested Types:

• When generating property or element read logic, if a nested type has a custom converter, the generated code now instantiates the custom converter and sets its GeneratedRead and GeneratedWrite delegates to point to the generated methods for that nested type. This ensures that custom converters can leverage the generated (de)serialization logic for their nested types. [1] [2]

• Similarly, when generating property or element write logic, the code now checks for a custom converter and, if present, creates an instance of it with the appropriate delegates wired up, instead of always using the generated converter directly. This is applied consistently for value types, nullable value types, and reference types. [1] [2] [3] [4]