feat: Threads sidebar

[matthewlipski]

This PR adds an optional sidebar for threads that can be used instead of having the threads float next to their respective marks. The ThreadsSidebar component can be imported from BlockNote to add the sidebar to any editor - see the docs for more.

Screen.Recording.2025-03-19.at.10.53.32-1.mov

BlockNoteView changes

The ThreadsSidebar component sits outside the editor but needs to have access to the editor context, including Mantine and theme variables. To do this, I modified what BlockNoteView and BlockNoteViewEditor render when renderEditor is false - here's an explanation for what's changed:

Previously:
BlockNoteView: Only provides context, doesn't render any elements.
BlockNoteViewEditor: Renders both bn-container and bn-editor elements.

Now:
BlockNoteView: Renders bn-container element.
BlockNoteViewEditor: Renders bn-editor element.

Since Mantine and theme variables are set on the bn-container element, this ensures that anything within BlockNoteView is styled properly. It's also worth noting that theme variables are used by the react package, and so we can't really make a similar change to e.g. the Mantine-specific BlockNoteView instead.

This change should not change anything when renderEditor is true or undefined.