cli: add --target flag

[landmaj]

closes #1707
closes #1727

This PR adds --target flag which allows exporting a single board from a multi-board diagram.

a -> b
layers: {
  x: {
    c -> d
  }
}

Layer "b" can be exported with one of the following commands:

d2 --target x index.d2
d2 --target layers.x index.d2

EDIT: comment updated to reflect changes after review.

[alixander]

I think in the future D2 should allow exporting just a container from a diagram, which can reuse this flag if we call it --render-target or something.

cc @gavin-ts any alt name suggestions?

[gavin-ts]

I think in the future D2 should allow exporting just a container from a diagram, which can reuse this flag if we call it --render-target or something.

cc @gavin-ts any alt name suggestions?

hmm maybe the target is implicit, I think I would prefer using d2 --render root.x index.d2 over d2 --render-target root.x index.d2

--render [target]
Render a single board from multi-board diagram. E.g. root.scenarios.x to render only scenario x. If not provided, the entire diagram is rendered

[alixander]

I don't think it's implicit. With just a --render flag, I'd think it's possible that it controls render format, or resolution. --render-target [path]

[landmaj]

It could also be just --target. This is how Docker handles building specific stages.

[alixander]

--target sounds good to me.

[landmaj]

PR updated to use --target flag.

[gavin-ts]

nice! looks good to me

resolved