Big refactor!

[danfishgold]

Oops this is a chunky one

I tried to combine all the changes I wanted to make to the reducer so that it all happens in one [massive] breaking change. This may have been a mistake.

I intend to keep the commits of this PR separate because they're not super-related.

Highlights

• moving the source code out of the reducer
• changing the markers and arrows so that they're stored in hash maps instead of arrays
• making the svg overlap only the code content rather than the entire area (which includes the line numbers and line markers)
• laying the groundwork for annotating code that doesn't come from a GitHub url
• moveing settings (straight lines, annotation brightness) to a separate context so that they're shared across different files (eventually, after I make them persisted)

[netlify[bot]]

✔️ Deploy Preview for crc-annotations ready!

🔨 Explore the source changes: d48e4177d52d5fee59eadc69346f71924a6e28ab

🔍 Inspect the deploy log: https://app.netlify.com/sites/crc-annotations/deploys/6183e6fcedbcc700084d4d69

😎 Browse the preview: https://deploy-preview-12--crc-annotations.netlify.app/