Typed Relation group by rel_type formatter

Problem/Motivation

I would like a formatter that can group typed relation values by `rel_type`. For example, instead of default "Typed Relation Formatter"

Contributors
Author: Lucy Maud Montgomery
Storyteller: Darcia Douglas
Author: Adam Globus-Hoenich

referenced entities would be grouped by `rel_type`, e.g.

Contributors
Author: Lucy Maud Montgomery, Adam Globus-Hoenich
Storyteller: Darcia Douglas

Proposed resolution

1. Check if anyone has already done this.
2. Create a new formatter, e.g. "Typed Relation Grouped Formatter"