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>

cmd/tailscaled/depaware-minbox.txt

@@ -55,7 +55,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/cmd/tailscale/cli                              from tailscale.com/cmd/tailscaled
         tailscale.com/cmd/tailscale/cli/ffcomplete                   from tailscale.com/cmd/tailscale/cli
         tailscale.com/cmd/tailscale/cli/ffcomplete/internal          from tailscale.com/cmd/tailscale/cli/ffcomplete
-        tailscale.com/cmd/tailscale/jsonoutput                       from tailscale.com/cmd/tailscale/cli
+        tailscale.com/cmd/tailscale/tsdnsjsonv0                      from tailscale.com/cmd/tailscale/cli
         tailscale.com/cmd/tailscaled/childproc                       from tailscale.com/cmd/tailscaled
         tailscale.com/control/controlbase                            from tailscale.com/control/controlhttp+
         tailscale.com/control/controlclient                          from tailscale.com/cmd/tailscaled+