Add 'heartbeat' to MPD client, so we notice if we disconnect

stubs/mpd/asyncio.pyi

@@ -9,6 +9,7 @@ class MPDClient(MPDClientBase):
 
 	def __init__(self) -> None: ...
 	async def connect(self, host: str, port: int = ...) -> None: ...
+	async def ping(self) -> None: ...
 	async def password(self, password: str) -> None: ...
 	def idle(self, subsystems: Sequence[str] = ...) -> AsyncIterator[Sequence[str]]: ...
 	async def status(self) -> types.StatusResponse: ...