Create the foundations of the design system

[salieflewis]

I'll create smaller tasks within this issue as we build towards a rudimentary design system

• [x] Rename the base directory to elements and include layout components like Stack
• [x] Create a Next.js playground for the design system inside our apps directory
• [x] Rename the design system itself, the package should appear as @river/estuary

cc: @ioeylim

[salieflewis]

In regards to icons, it looks like what Mirror does, is import the actual svg files, and then run a generate script which leverages this to turn those files into React components.

[ioeylim]

This is all great!! I'm excited for this

[salieflewis]

Resolved #91