Redesign nested layout

[gavin-ts]

Summary

Redesign nesting of different layout types (grid, sequence diagram, nears).

Details

• redesign supports more flexible nesting, and central nested graph/diagram type management
• will support nested edges in grids and simple edges across diagram types
• now supports sequence diagrams within grid (TODO including edges see below)
• support grids in sequence diagram actors (TODO compiler err against nesting in groups/notes)
• can be adjusted to enable constant nears in any container instead of only at the root level
• TODO remove compile validation against nested edges in grids
• TODO test with nested edges in grid/sequence (nested edges follow up PR nested edges within grid cells #1629)
• adds nested_diagram_types test
• fixes order of shapes/edges in some tests with constant nears / sequences
• TODO consider supporting dagre in elk or elk in dagre

nested diagram types