chore: Reconfigure @typescript-eslint/explicit-function-return-type

[rekmarks]

Enables the allowExpressions config option of the ESLint rule @typescript-eslint/explicit-function-return-type. See rule documentation here. This has the following effect:

// These were already allowed
node.addEventListener('click', () => undefined);

node.addEventListener('click', function () {});

const foo = arr.map(i => i * i);

// Newly allowed
const obj = {
  foo: (bar: string) => bar,
}

// Still forbidden
const fn = (bar: string) => bar;

This is motivated because this rule was disabled here in #37, and I think the rule is excessive in such cases.

Incidentally, removes an // eslint-disable directive by replacing an empty function () => {} with () => undefined.

[SMotaal]

@rekmarks This is not a blocker, just curiosity about the reasons for this?

As far as the runtime is concerned, they are equivalent, as long as the runtime is optimizing for no-op returning undefined.

[rekmarks]

We should prefer code of equivalent behavior and complexity over lint exceptions when possible. Lint rules exist for a reason, and the more exceptions we add, the more we compromise the stylistic and behavioral patterns we want to enforce with our lint rules. Lint exceptions should therefore only be used when necessary.

[SMotaal]

LGTM