fix: external trampoline type checking

[nicklafleur]

Addresses issues introduced in #476

Changes:

• Detects the Self annotation and replaces it with generics in mutated functions to support type checking
• Normalize external trampoline naming conventions to the established patterns (this had some side-effects with type checking as well)

Tests:

• add tests that validate inheritence of class/static methods passes type checking with the generic replacement

Ref: c866681

class Color(Enum)
    @classmethod
    def create(cls, name: str) -> Self:
        return cls(name)
...
_TxǁColorǁ = TypeVar('_TxǁColorǁ', bound='Color')

def xǁColorǁcreate__mutmut_orig(cls: type[_TxǁColorǁ], name: str) -> _TxǁColorǁ :
    return cls(name)