cmd/tailscale/tsdnsjsonv0: extract a new package for `tailscale dns --json` (#20017)

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>