build(IntelliJ): turn on checkstyle.scan-before-checkin

[keturn]

Small tweaks to how CheckStyle is setup in gradle and IntelliJ.

The most impactful one is that scan-before-checkin.

I was tempted to see if we could get rid of the samedir property, but that princess is in another castle.

[keturn]

* What went wrong:
Execution failed for task ':engine:checkstyleJmh'.
> Unable to store input properties for task ':engine:checkstyleJmh'. Property 'configProperties' with value '{samedir=extension 'checkstyle' property 'configDirectory'}' cannot be serialized.

[keturn]

fixed. There was some groovy/gradle typing hijinks going on that threw a wrench in the first attempt.