Formatter: `blank_line_after_nested_stub_class` preview style

[MichaReiser]

Implement Black's blank_line_after_nested_stub_class as a preview style. This new style only applies to stub files (.pyi)

class Outer:
    class InnerStub: ...
    outer_attr_after_inner_stub: int
    class Inner:
        inner_attr: int
    outer_attr: int

# output
class Outer:
    class InnerStub: ...
    outer_attr_after_inner_stub: int

    class Inner:
        inner_attr: int

    outer_attr: int

See added new lines after the nested classes (that don't have a dummy body)