Suggest code actions if the Bake file references a build stage that does not exist

[rcjsuen]

FROM scratch AS stage
FROM scratch AS stage2

target "default" {
  target = "stage3"
}

We should suggest:

1. changing stage3 to stage or stage2 by scanning for all stages in the referenced Dockerfile
2. creating a new FROM line that will be named stage3

1 is straightforward. 2 may be questionable as we'd need to introduce a WorkspaceEdit for another file. We should review what the UX will look like before necessarily committing to implementing 2.