wip: sequence diagrams v2

[alixander]

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

1. Solve better vertical control

• perhaps reuse grid-gap to specify sequence diagram vertical gaps #1738
• Better vertical control on sequence diagram #1183

2. Span labels

• [sequence diagram] label spans #2568

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

• d2sequence: Fix sequence_diagram_edge_group_span_field #748
• Undeclared actor within sequence_diagram group causes weird problems #1557

4. Switch sequence_diagram to sequence-diagram

• consider standardizing - and _ #632

5. Replication of actors midway/bottom

• [Feature] Sequence Diagram Visualization with Bottom Replication of Actors #1553

6. Group actors

• Feature request: grouping actors in sequence diagrams #1320