This PR implements Bear-style heading & list folding using invisible HTML tags that match Bear's. The state of folding in the document is persisted in the document itself, with the following pros & cons:

pros:

• fold state is persisted for no additional complexity
• fold state is invisible to markdown renderers
• copy & paste preserves state even from lockbook to bear and back (2nd video demo below)
• undo applies to fold changes in case something unexpected happens

cons:

• fold state is not invisible to markdown editors, shows up as inline HTML in Obsidian (screenshot below)
• affects suggested docs & activity
• incurs network overhead
• fold state is synced across users
• requires write access to a document to use

fixes: #3934

There's some kind of funkiness with the selection - when folding an element, if the cursor is below the element, the editor will scroll to the cursor.