given some plain text, e.g.: "The quick brown fox" allows you to apply
multiple spans at multiple positions. Positions can be specified by
word, that is e.g. "brown" or range (from=10, to=15) and spans are
subclasses of InlineSpan
Once you are done using SpanBuilder, use build to return calculated list of spans.
The rest of the text, that has no span either inherits TextStyle from parent span or can be forced
to use defaultStyle