- Remove use of `kwargs` in setting `cls` if it's given as `ClassDefinition`
- Correct detection of duplicating classes
- Ensure `slots` is correct type when `use_attributes=True`

The example is outdated since `SchemaBuilder` no
longer resides in `linkml.utils.schema_builder`

In this way, the slot definition in `slots` can be added
to the class as an attribute, better than raising an
exception

The bundling is actually not needed. The type
annotation of `slots` already ensures that it is
a list or `None`

Test the case of adding a class with a name that is
the same as a class that already exists in the schema

… dict

This can simplify the code

…tion`

This time with detection of unsupported fields provided by
`kwargs`