Add naive feedback highlight system

[connorff]

Overview

This PR adds a (currently) naive system for adding and persisting feedback on book content.

Video example

https://user-images.githubusercontent.com/40175891/182223568-45bab38c-b625-47a6-b2b4-fc32dc67fc80.mp4

Implementation

The feedback.tsx entrypoint instantiates a SelectionRenderer which does a couple things:

1. Renders any feedback currently stored in local storage as a yellow highlight
2. Tracks the user's current selection range (what they're highlighting in the document) and renders a tooltip over the selection
   • The tooltip component is found in tooltip.tsx
   • Clicking on the question mark icon in the tooltip triggers a modal for giving feedback (component in modal.tsx)
3. Stores new feedback in local storage when created
4. Displays a tooltip over existing feedback highlights when they're hovered over

Serialization of feedback ranges is done by web-highlighter. Their serialization strategy does not seem well-suited for dynamic content, something that will need to be fixed for a production-ready version of this feature.

Additional considerations

• This PR does not currently send feedback to the telemetry/results server
• This PR does not allow users to delete feedback (is this something that should be added?)

Todo

• [x] Replace hard-coded colors with theme variables
• [x] Factor out SelectionRenderer into multiple, simpler components
• [x] Add ability to delete a highlight
• [ ] Add integration test
• [ ] Fix issue where not adding feedback causes the tooltip to render a question mark

[willcrichton]

Closing this and moving everything to the rust-book repo.