Add geom_utterance

[bvreede]

Instead of text integrated in the geom_turn (see #26), it would be good to have a separate geometry that allows plotting the utterance text.

It would inherit y (participant), begin and end, and provide additional aesthetics utterance for the utterance text.

Possible inspiration in this vignette for GeomTextPath.

[mdingemanse]

Useful mental model for how to deal with overflow (when utterance content would fall outside the turn bounds) may be the CSS overflow property: https://www.w3schools.com/css/css_overflow.asp

Some combination of text wrap and overflow options, with sensible defaults, might go a long way.