In PR #19641, we added the `tailscale routecheck` command that supports both `--format=json` and `--format=json-line`. To allow other packages to import and unmarshal that JSON structure, this patch exports that format in a new tsroutecheckjsonv0 package. Updates #17366 Updates tailscale/corp#33033 Signed-off-by: Simon Law <sfllaw@tailscale.com>