Create eslint rule to enforce function return type #3777

Right now it is perfectly fine to write a function like this:

private doSomething() {
   // do some stuff
   //  ...
   return result;
}

Best practice is to always define a return type for your function. We could enforce this with an eslint rule. The rule we are looking for is @typescript-eslint/explicit-function-return-type.

We should enable the eslint rule and fix all the functions that violate this rule.