fix: generate correct override stub for interface when implementation from base class

[HerrCai0907]

when class implements interface by its base class member, the original design will not manage it.
For example,

interface I {  foo(): i32; }
class Base {
  foo(): i32 { return 42;}
}
class C extends Base implements I {}

class C implements interface I, but when creating I#foo override stub, it will only search the member of class C. which does not include Base#foo.

This PR wants to split override (by extends) and implementation (by implements). Handling override needs to check whether the current class overrides the base class method. Handling implementation needs to check whether the current class and its base class implements the interface method.

[CountBleck]

I'm not sure if I can say anything with confidence one way or another, to be honest. I'm not that familiar with how propagation across classes/interfaces is supposed to work...