Add checkstyle plugin to common.gradle
Add a checkstyle.xml based on google checkstyle xml with ammendments.
Checkstyle errors do not fail the build, everything is marked as a
warning, can be refined over time as per comments in jMonkeyEngine#1877.

Checkstyle can be run with `./gradlew checkstyleMain` - it runs for all
projects.

Wound back checkstyle version to 9.3
Last good version to work out of the box with java 8
Re-copied the google checkstyle config from that version too
Updated the links in the checkstyle config to the right docs url.
Rebuilt with Java 8 & 17 to test.

* Ignore some modules that produce a lot of warnings.
* Increase line length to 150 so we only get the worst offenders
* Reduces output by ~70% (counting main.html sizes, about 4M)

* Don't run checkstyle on jme3-networking and jme3-vr
* These two make up about 1M of reporting
* Fixed ref to checkstyle-suppressions and added a template file
* Removed suppressionXPathFilter
* Disabled a couple more checks.
* Output is down to ~ 2.2M

* Only process /com/jme3/renderer sources
* Rolled back jme3-network/vr exclusion (still not processed though)
* Added modules back into checkstyle config
* Now gives 38 errors from 11 files within com/jme3/renderer package
* BeforeExecutionExclusionFileFilter wasn't playing nicely

* Realised I made a mistake on the package filter
* was only pulling java classes directly under com/jme/renderer/
* Ammded to include all java classes in that package
* Opened up to 200Kb of errors over 4 projects

* Made style changes suggested in the PR comments