Emit close events from WebSocketProvider

[afurm]

Summary

Fixes #4587.

This makes WebSocketProvider surface underlying websocket close events through the provider event API.

Changes

• Adds optional onclose support to WebSocketLike
• Emits provider "close" when the underlying websocket closes
• Allows "close" as a valid provider event
• Documents the "close" provider event
• Adds a regression test for websocket close emission

Testing

• npm run build-all
• npx mocha --trace-warnings --reporter ./reporter.cjs ./lib.esm/_tests/test-providers-websocket.js
• npx mocha --reporter ./reporter.cjs ./lib.commonjs/_tests/test-providers-websocket.js