cmd/tailscale/cli/jsonoutput: extract routecheck’s output format

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>
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