Assert the read/write expect state of each element of the API

[oleiade]

It was mentioned during internal discussions that we might want to enforce the read-only state of certain properties exposed by the module. We should careful assert and test this property of the API in a test suite.

It was mentioned that a "trick" exists to enforce that in a Goja context: https://github.com/grafana/xk6-websockets/blob/main/websockets/websockets.go#L133-L157