Mark experimental rules using interface "Rule.Experimental"

* Add interface "Rule.Experimental" to mark a rule as an experimental rule. This allows experimental rules to be defined in any rule set. Whenever a rule is no longer experimental it does not need to be moved to a non-experimental rule set. Benefit is that users of that rule, do not need to change ktlint disable directives.

* Merge `experimental` ruleset with `standard` ruleset.

* Rearrange standard rules to separate package

* Simplify code by moving logic from QualifiedRuleId to Rule. All logic regarding qualified rule (set) id is now kept together in once class.