Persistent sidebar collapse using middleware and async-provider

[brookslybrand]

Persistent Sidebar Collapse

This features retains each user's open/collapse preferences for the sidebar using:

• Cookies
• Middleware
• New context

Collapse states work with SSR (no flash of collapse/expand). Persisting the collapse state only works once JS has been loaded (progressively enhanced)

How does it work

• middleware reads the value from the cookie (if it exists) and loads it into context
  • Middleware is also responsible for setting the cookie
  • Cookie lasts 1 week -- if a user hasn't been on the site in a bit, having everything open isn't a bad thing
• The docs-layout route reads the context in the loader and passes it down as loaderData
  • All sections default to open if there's no value set
  • Only use the user's preferences on the main docs -- keeping track of dev, v6, and every other tag is kind of unnecessary
• When the user toggles, they trigger an action. The action updates the context
• middleware, after processing the request from the action, sets the new context to the cookie

Demo

persistent-collapse.mov

What else does this PR change

I got a little carried away and refactored the color scheme changer, just to match the structure of the code I added.