Target spec JSON files are not checked for unknown keys

[phil-opp]

Target JSON files are currently not checked for unknown keys, so typos can easily lead to bugs. For example, a "panic_strategy": "abort" key is silently ignored because the correct key name is panic-strategy (- instead of _).

This is not a new problem, but I could not find any open issue for this. Feel free to close this if this is a duplicate.

[xzmeng]

After #85775 it emits a warning if there is any unknown key.