[feature request] box arrowhead and component shape

[pmoura]

I have a diagrams tool that generates diagrams using Graphviz (simple example). I'm updating it to also support d2. It would be possible to add native d2 support for box arrowhead and the component node shape?

Thanks for your work in d2 and looking forward to some of the upcoming features being worked on (notably, links in edges).

[alixander]

Can you make these into separate issues that detail what each one is? Like requirements of what fulfills a "component" shape

[pmoura]

As requested:

#2227
#2228