Adds checkstyle

[StrongestNumber9]

This is WIP as not all check categories have been sanity checked and some checks might need to be added/removed from the current list as some stuff like MagicNumbers can be very painful to deal with

The following check subcategories have not yet been checked:

https://checkstyle.org/checks/annotation/index.html - might have no need?

https://checkstyle.org/checks/header/index.html - Enforced already by formatter plugin

https://checkstyle.org/checks/javadoc/index.html - These might be good to add

https://checkstyle.org/checks/metrics/index.html - Questionable

https://checkstyle.org/checks/naming/index.html - These require a lot of thought, might be good idea to add in a new PR if adding it?

https://checkstyle.org/checks/regexp/index.html - Might be good to add stuff like checking for System.out.println and such calls

https://checkstyle.org/checks/sizes/index.html - Also questionable as some of the checks comes from human code reviews

https://checkstyle.org/checks/whitespace/index.html - Should be enforced by formatter already

[kortemik]

ban System.out all together?

[kortemik]

see also https://github.com/yegor256/qulice/blob/master/qulice-checkstyle/src/main/resources/com/qulice/checkstyle/checks.xml

[StrongestNumber9]

ban System.out all together?

Sounds useful for any projects that use logger I guess. Or just remove the rule if non-logger printing is used