Add a Mercure connector

[dunglas]

Companion PR for laravel/framework#61474

It adds a connector for the Mercure broadcast driver.

It relies on the native EventSource API: no client library to install, and the hub URL defaults to /.well-known/mercure on the current origin.

Features:

• One EventSource multiplexes every joined channel; joins and leaves coalesce into a single re-auth and reopen.
• Auth uses the batch /broadcasting/auth endpoint and an httpOnly cookie, re-minted before the reported expires_in elapses.
• Presence channels are seeded from the hub's subscription API and updated live from subscription events.
• End-to-end encrypted channels decrypt compact JWEs with WebCrypto, using the per-channel JWK returned by the auth endpoint. Plaintext updates targeting an encrypted channel are dropped.
• Whispers publish directly to the hub on per-channel whisper topics and arrive on a second, whisper-only EventSource that dispatches client-* events exclusively, so channel members cannot forge server events. On encrypted channels, whispers are sealed under the channel key before leaving the sender.
• Config presets for the React, Vue, and Svelte packages (VITE_MERCURE_HUB_URL).