Simple edges across nested diagrams

[gavin-ts]

Summary

Edges can now be created across different nested diagram types and grid cells.

Details

• follow up to nested edges within grid cells #1629 which supports nested edges and therefore more types of nesting
• review after nested edges within grid cells #1629 to see only this PR's changes
• updates nested layout to implement simple edge routing across different nested graph types
• edges between grid cells of different grids, or descendants of different grid cells are now supported
• edges into a constant near or sequence diagram are now also supported
• adds grid_edge_across_cell test
• adds nesting_power test making use of Redesign nested layout #1602's new design to implement these edges across-nested graph types
• TODO eventually replace simple straight-edge routing with improved routing to avoid overlaps

edges between descendants of different grid cells are now supported

nesting_power test showing edges across nested diagrams (green)