TypeScript: Use with Jest

[swissspidy]

Let's use ts-jest for improved TypeScript support in Jest so that we can also write unit tests with TypeScript —with proper type checking and not just transpilation.

See https://kulshekhar.github.io/ts-jest/docs/babel7-or-ts/ See https://kulshekhar.github.io/ts-jest/docs/27.0/guides/test-helpers/

• [ ] Update ESLint config
• [ ] Add preset: 'ts-jest' to Jest config