Replace socket.io-client imports with a local type shim in echo.d.ts

[lazerg]

Since v2.3.7, the compiled dist/echo.d.ts unconditionally imports Socket and io from socket.io-client. Users who only use Reverb or Pusher never install that package, so TypeScript fails on those imports when skipLibCheck is false.

The root cause is that socketio-channel.ts and socketio-connector.ts import from socket.io-client, and vite-plugin-dts (with rollupTypes: true) inlines those module references into the rolled-up declaration file.

This PR adds a minimal local type shim (src/socketio-types.ts) that declares the small subset of Socket.io types Laravel Echo actually uses — SocketIoSocket, SocketIoManager, and SocketIoFunction. The two connector/channel files and typings/window.d.ts are updated to import from the shim instead of from socket.io-client. The real socket.io-client types satisfy these interfaces, so nothing changes for users who do have the package installed.

After this change, dist/echo.d.ts no longer contains any import … from 'socket.io-client' line. Verified by temporarily removing the package from node_modules and running tsc --noEmit — no errors. All existing tests still pass.

Fixes #528