add support for static variables in Check DSL

[rursprung]

the Check DSL currently does not allow defining static variables. support for this should be added to improve the performance (e.g. a java.util.regex.Pattern should be compiled only once rather than for each check execution).

a workaround exists: the static variable can be defined in a separate java helper class and then be referenced from within the Check DSL. however, this is an overkill if no such helper class exists yet and that static variable would be its only content.