chore: enable no-floating-promises eslint rule

This MR introduces eslint rule that ensures we handle every Promise we create in the code. Without this rule, it's possible to call a function that returns a Promise without awaiting the Promise.

Documentation: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-floating-promises.md

Why?

Even though it doesn't happen often, leaving a floating promise in the code leads to strange behaviour in both production code and tests.

Related

3 months ago we added return-await rule that makes sure that we have to await promises in try/catch blocks.