a8n: Add example showcasing usage of ESLint to upgrade to new TypeScript syntax

Created by: mrnugget

This adds the last example @sqs wrote for the new src-cli to the docs.

I'm not 100% about the instructions here (even though I copied Quinn's Dockerfile as is) since it takes a really long time to run on my machine.

@sqs @sourcegraph/web Maybe you can take a closer look at the wording here ("rules", "problems", etc.)