d2plugin: Sketch plugin

[alixander]

Also implements being able to run D2 with multiple plugins, by adding a Options() on d2plugin interface that returns the list of operations the plugin supports.

See https://github.com/terrastruct/d2/pull/45 for more discussion

[nhooyr]

Update the changelog please.

[nhooyr]

And manpage.

[nhooyr]

And remember this needs to address my review in https://github.com/terrastruct/d2/pull/45#pullrequestreview-1170967901 before it can be merged.

[nhooyr]

think we can get this in for v0.0.14 on monday?

[alixander]

unlikely. there's actually quite a lot here, we'll get it for the release after

[alixander]

closing in favor of this: https://github.com/terrastruct/d2/pull/492

(we don't use v8go so no need for it to be a plugin anymore)