Node kind targets for theming

[StoneCypher]

These are things that should show up for style support.

1. &_state (rounded rect)
2. &_start_node (hard rect with outer border)
3. &_end_node (hard rect with outer border)
   1. final
   2. terminal
   3. complete
4. &_decision (diamond)
5. &_procedure (hard rect with sidebars)
6. &_spawn (hard rect)
7. &_input (parallelogram)
8. &_output (crt shape - <_))
9. &_loop (top half of octagon)
10. &_looplimit (bottom half of octagon)
11. &_storage (cylinder)
12. &_error (trapezoid \__/)
13. &_main_path
    1. this is when a node has at least one => entrypoint
14. &_forced_path
    1. this is when a node has only ~> entrypoints

Similarly, there should be style sets for implicit edge groups

1. &_edge
2. &_main_edge
3. &_forced_edge
4. &_end_edge
   1. &_final_edge
   2. &_terminal_edge
   3. &_complete_edge
5. &_start_edge

Also, for the graph itself

1. &_graph

See #114 #285 #439 #338 #292 etc

[StoneCypher]

Love the feature. Rethink the notation.

Consider these to be default styles, which means they'd fit the style cascade #114 and be useful in themes #285