Enable type-aware linting and fix a couple issues

[controversial]

Stacked on #331; ignore all but the top two commits: d270525 b1f7eb1

Replaced @typescript-eslint/recommended with @typescript-eslint/recommended-type-checked, and fix a few issues that were raised by the additional rules:

• Some places where a value whose type was like string | { name: string } was passed into a template literal, resulting in possible edge cases where we’d get [object Object] in the template literal instead of the intended name
• Parse/validate the shape of package.json before reading it for improved type safety