✨ Extension Store

# Overview
The extension store should allow users to browse and install extensions submitted by other developers. They should be categorized using the information provided in each extensions manifest.json (See https://github.com/CodeEditApp/CodeEdit/issues/174)

## Tasks
- [ ] Determine storage requirements
- [ ] Design extension store UI (#355)
- [ ] Build extension submission process
- [ ] Build library, using information in submitted extension manifest files.

## Concept

I'd imagine when one is clicked a tab in the main content area opens up to show more details.