[ty] Playground: Better default settings (#18316)
## Summary
The playground default settings set the `division-by-zero` rule severity
to `error`. This slightly confusing because `division-by-zero` is now
disabled by default. I am assuming that we have a `rules` section in
there to make it easier for users to customize those settings (in
addition to what the JSON schema gives us).
Here, I'm proposing a different default rule-set (`"undefined-reveal":
"ignore"`) that I would personally find more helpful for the playground,
since we're using it so frequently for MREs that often involve some
`reveal_type` calls.
