heads up: Enabling Java checkstyle (again)

[Tomohisa Igarashi]

TBH I can't remember, when/why it's disabled, how we were doing it before, etc.

Anyway re-enabling Java checkstyle
https://github.com/atlasmap/atlasmap/pull/2192

It also adds "checkstyle:check" to CircleCI build script, so checkstyle runs against PR check.
But it's not mapped to regular maven phase, so you need to explicitly run "mvn checkstyle:check" to locally run it.

Thanks,
Tomo