Support `sys.implementation.name` as feature switch for stubs

Problem

Sometimes, attributes and variables are only defined for a specific implementation.
E.g. I recently saw that PyPy has a sys.pypy_version_info variable.

At the moment, type checkers only support sys.version_info and sys.platform checks AFAIK.^1

Idea

Add support for sys.implementation.name^2 checks.

import sys

if sys.implementation.name == "pypy":
    pypy_version_info: ...

首先查看 typing 仓库，以及 issue 中对 Python 的 sys.implementation.name 和 PEP 421 的引用。确定实现特定的功能开关在哪里指定或测试；当能够识别诸如 sys.implementation.name == "pypy" 之类的检查，从而正确处理 pypy_version_info 之类的条件定义时，即视为完成。