Don't include Learning Assistant in the bundle

To reduce the bundle size for everyone, we need to make a few fixes to https://github.com/edx/frontend-lib-learning-assistant :

1. It's peerDependencies are overly specified. For example, it shouldn't pin redux to the exact version 4.1.2 as that blocks us from bumping the version in frontend-app-learning. Specifying ^4 or ^4.1 is fine.
2. It should not be part of frontend-app-learning's redux store. Instead it should have its own store and its own <Provider>. Or better yet, don't use redux at all.
3. The <Xpert> component inside <Chat> should use React.lazy so that the learning-assistant code is only loaded on demand, and not always bundled into the main learning MFE app bundle.
4. Optional: It should be a plugin using frontend-plugin-framework, and not even mentioned in the code at all.

Numbers 2 and 3 are the main thing I'd like to achieve here, as having both of those will result in a bundle size reduction.

Start by reading src/store.js and src/courseware/course/chat/Chat.jsx, especially the Learning Assistant store entry and the Xpert component usage. Review the linked Learning Assistant package.json peerDependencies as context. Done means the assistant is isolated from the app store and loaded on demand so it is no longer included in the main bundle.