* Upgrade to Yarn 3.x
* Move test files into `src/` instead of keeping them in `test/`
* Convert Jest config file back to JavaScript
* Add `eslint-plugin-jest` to lint pipeline, and correct issues
* Extract build-only config from `tsconfig.json`
* Standardize `package.json`
* Don't add `jest-it-up` to the `test` command because there is
currently an inconsistency between different Node versions (14.x and
16.x) which may have to do with global `fetch`
* Upgrade `@metamask/allow-scripts` to 2.x
* Upgrade `@metamask/utils` to ^3.0.3
