Update TypeScript ESLint packages

[whymarrh]

Prereq for #268

This PR updates the @typescript-eslint/* packages, introducing support for TypeScript 4.

[whymarrh]

The switch away from object types is in response to the updated ban-types rule:

 45:9 error Don't use `object` as a type. The `object` type is currently hard to use ([see this issue](https://github.com/microsoft/TypeScript/issues/21732)).
Consider using `Record<string, unknown>` instead, as it allows you to more easily inspect and use the keys @typescript-eslint/ban-types

[Gudahtt]

LGTM!