Function isPortableTextToolkitSpan

isPortableTextToolkitSpan(span): span is ToolkitNestedPortableTextSpan<PortableTextMarkDefinition>
Loose check to determine if span is a toolkit span node. Only checks _type, assumes correct structure.
Parameters
- span: TypedObject | ToolkitNestedPortableTextSpan<PortableTextMarkDefinition>
  Span to check
Returns span is ToolkitNestedPortableTextSpan<PortableTextMarkDefinition>
True if toolkit span, otherwise false