types(client): add and export ExtendedError for connect_error event

[bhumin18]

Description

This PR adds and exports the ExtendedError interface in socket.io-client and updates SocketReservedEvents to type connect_error: (err: ExtendedError) => void.

Motivation

When a connection middleware rejects the connection with extra metadata, the client receives PacketType.CONNECT_ERROR and sets err.data = packet.data.data. Previously, connect_error was typed as standard Error, which required TypeScript users to write (err as any).data.

This change mirrors the server package's ExtendedError declaration, enables type-safe access to err.data, and removes the need for // @ts-ignore in onpacket.

Fixes #5542

Checklist

• TypeScript builds successfully (npm run compile --workspace=socket.io-client)
• Code matches Prettier style (npm run format:check --workspace=socket.io-client)
• Unit tests and type tests added and passing

[bhumin18]

Hi @darrachequesne, just checking in to see if any additional tests or changes are needed for this PR whenever you have a moment to review. Thanks!

[darrachequesne]

@bhumin18 nice, thanks 👍