Type is always span for portable text spans, as these don't vary in shape
The actual text value of this text span
Optional _keyUnique (within parent block) key for this portable text span
Optional marksAn array of marks this text span is annotated with, identified by its _key.
If the key cannot be found in the parent blocks mark definition, the mark is assumed to be a
decorator (a simpler mark without any properties - for instance strong or em)
A Portable Text Span holds a chunk of the actual text value of a Portable Text Block