Fix the JumpNb feature

[Fenrisfulsur]

Fix the JumpNb feature by replacing mnemonic check with capstone group check

[patacca]

Thanks for the contribution, I just changed two things:

• avoid importing modules when needed only for type hints, instead import them under the if TYPE_CHECKING condition. This has not been enforced on the entire code base yet but let's try to avoid this practice on the new code

• Wrap the casting from capstone group into a specific method, this should help for code reuse and it can centralize the conversion from capstone into a single place. If in the future we will have misalignment issues it will be easier to address them.

  Beside this LGTM!