New check request: Unreachable methods in anonymous classes

Any non-override, non-private method in an anonymous class is at least "dodgy" and might fool a dead code detector.

Non-override methods are hard to reach from outside the class body itself. There are two kinds of anonymous class:

* In a "class instance creation expression with class body", a non-override method is only reachable one of two ways:
* calling it directly with the anonymous class expression itself as the receiver (`new Object() { void foo() {} }.foo()`)
* if the anonymous class expression is assigned to a local variable that uses `var` instead of an explicit type
* In an "enum constant with class body", a non-override method is not usable from outside that body at all.

If such a method is marked public, that might prevent a dead-code detector from recognizing it can be cleaned up. It would probably be a good practice to always make it private. Unfortunately, those two weird cases for non-enum anonymous classes should probably be exempted.

No idea if this is worth investigating further, just realized this as another strange wart in the language.