Introduce new WebSockets receiver impl

When enabled, this new receiver will spin up a local WebSockets server
and will send the currently playing song information to any clients that
connect. It's designed with Übersicht in mind, since WebSockets is the
easiest way to efficiently push events into an Übersicht widget, but
I'm sure it'd work for a variety of other purposes too.

Currently the socket is only used in one direction, pushing the current
song info from server to client, but I'll probably extend it to support
sending MPD commands from WebSockets clients as well.

src/mpd_now_playable/receivers/websockets/receiver.py

@@ -0,0 +1,52 @@
+from pathlib import Path
+
+import ormsgpack
+from websockets import broadcast
+from websockets.server import WebSocketServerProtocol, serve
+
+from ...config.model import WebsocketsReceiverConfig
+from ...player import Player
+from ...song import Song
+from ...song_receiver import DefaultLoopFactory, Receiver
+
+MSGPACK_NULL = ormsgpack.packb(None)
+
+
+def default(value: object) -> object:
+	if isinstance(value, Path):
+		return str(value)
+	raise TypeError
+
+
+class WebsocketsReceiver(Receiver):
+	config: WebsocketsReceiverConfig
+	player: Player
+	connections: set[WebSocketServerProtocol]
+	last_status: bytes = MSGPACK_NULL
+
+	def __init__(self, config: WebsocketsReceiverConfig):
+		self.config = config
+		self.connections = set()
+
+	@classmethod
+	def loop_factory(cls) -> DefaultLoopFactory:
+		return DefaultLoopFactory()
+
+	async def start(self, player: Player) -> None:
+		self.player = player
+		await serve(self.handle, host=self.config.host, port=self.config.port)
+
+	async def handle(self, conn: WebSocketServerProtocol) -> None:
+		self.connections.add(conn)
+		await conn.send(self.last_status)
+		try:
+			await conn.wait_closed()
+		finally:
+			self.connections.remove(conn)
+
+	async def update(self, song: Song | None) -> None:
+		if song is None:
+			self.last_status = MSGPACK_NULL
+		else:
+			self.last_status = ormsgpack.packb(song, default=default)
+		broadcast(self.connections, self.last_status)