Consider marking the ws peer dependency as optional

[NoahAndrews]

According to the peerDependencies documentation, NPM 7 will auto-install peerDependencies if they aren't explicit dependencies of the application. This means that an in-browser application that depends on this library will end up downloading the ws library even though it's not needed.

In my opinion, we should mark ws as optional using peerDependenciesMeta, and document that node.js applications should make sure to include ws as one of their dependencies. We'll probably also want a runtime check that the ws library is present, and throw an Error with a helpful message if it's not.