flatten

Flattens the list of KoTypeArgumentDeclaration objects into a list of KoBaseTypeDeclaration objects.

This method recursively extracts all the type arguments and their source type declarations, producing a flat list that includes both the outer types and their nested generic types.

For example:

• For a type argument like String, it returns listOf().

• For a type argument like List<String>, it returns listOf(String).

• For a type argument like Map<List<String>, Int>, it returns listOf(List, String, Int).