Fail the build on wrong import order

The XWiki Java code style documents an import order (java, javax, jakarta, org, com, anything else, then static imports, each group separated by a blank line and sorted alphabetically), and the IntelliJ IDEA and Eclipse settings we ship produce exactly that order, but nothing verifies it at build time.

As a consequence:

• A file whose imports are in the wrong order produces a spurious diff as soon as somebody with a properly configured IDE touches it.
• The order has to be corrected by hand during reviews, both for contributions from non-committers and for code written by coding agents.

Enable Checkstyle's ImportOrder check so that a wrong import order fails the build, for main sources (checkstyle.xml) as well as for test sources (checkstyle-test.xml).

Note that this applies to every project inheriting the XWiki parent POM, including xwiki-contrib extensions.

See the proposal on the forum.