This patch extracts all the DNS related JSON handling from the cmd/tailscale/jsonoutput package into a new tsdnsjsonv0 package. It adds package documentation for tsdnsjsonv0 with a big WARNING that this is an unstable format with no backwards compatibility guarantees. When we stabilize this format, we should spin off a new tsdnsjsonv1 package that uses jsonoutput.ResponseEnvelope to declare version 1. Updates #13326 Updates #18750 Signed-off-by: Simon Law <sfllaw@tailscale.com>