Consider enabling explicit-module-boundary-types

[jaworek]

Describe the feature

Explicit types for function return values and arguments makes it clear to any calling code what is the module boundary's input and output.

https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/explicit-module-boundary-types.md

This might be a big change for some of the projects that use this config, so it should be considered as breaking change.

Motivation

It improves debugging and makes sure that returned types are correct.

Related Issues

jestjs/jest#9539