wip: sequence diagrams v2

[alixander]

Aim to rewrite sequence diagram implementation and include many improvements in the rewrite.

1. Solve better vertical control

• https://github.com/terrastruct/d2/issues/1738
• https://github.com/terrastruct/d2/issues/1183

2. Span labels

• https://github.com/terrastruct/d2/issues/2568

3. Explicit edge groups, which remove the requirement that actors need to be predefined. This was always a bad language quirk.

• https://github.com/terrastruct/d2/issues/748
• https://github.com/terrastruct/d2/issues/1557

4. Switch sequence_diagram to sequence-diagram

• https://github.com/terrastruct/d2/issues/632

5. Replication of actors midway/bottom

• https://github.com/terrastruct/d2/issues/1553

6. Group actors

• https://github.com/terrastruct/d2/issues/1320