Adds outline to sidebar

[cindywu]

This PR

• adds outline component to sidebar using post headings
• matches minimap and post editor styles and ratios
• polishes minimap editor styles

Design: https://www.figma.com/file/gt3p3JDh5W5i5N2wXxE7I5/Design-System?node-id=76%3A86

• [x] refactor .Prosemirror styles in defaults.scss
• [x] refactor sidebar.scss
• [x] polish all #sidebarEditor styles
• [x] update sidebar icon and add sidebar mode labels
• [x] add outline sidebar component
• [x] update sidebarToggle from two modes to three (none, minimap, outline)
• [x] create outline component
• [ ] write plugin to add id to heading nodes using the title
• [ ] truncate heading text
• [ ] move minimap code out to minimap component
• [ ] add id to each heading in a post
• [ ] clicking heading in sidebar scrolls user to heading in post editor
• [ ] clicking heading in sidebar updates url with id (user should be able to copy url from browser and share and recipient will land at the desired heading)
• [ ] default to none (sidebar icon only)
• [ ] polish ui (only show sidebar label on hover, dissolve sidebar when typing)

[linear[bot]]

JEL-262 add outline using headings to sidebar

[cindywu]

• [x] update sidebarToggle from two modes to three (none, minimap, outline)

[cindywu]

closing this PR. this PR got broken into parts... part 1 is ready for review https://github.com/jellypbc/poster/pull/365