Add no-floating-promises as default rule

[yamadayuki]

WHY & WHAT

We can now use the no-floating-promises
typescript-eslint/typescript-eslint#495

This rule forbids usage of Promise-like values in statements without handling their errors appropriately. Unhandled promises can cause several issues, such as improperly sequenced operations, ignored Promise rejections and more. Valid ways of handling a Promise-valued statement include awaiting, returning, and either calling .then() with two arguments or .catch() with one argument

How do you think about this?
@KentoMoriwaki @chloe463 @kobayang

[KentoMoriwaki]

I want this rule enabled by default.

It is very useful especially when writing tests which including async functions.
For example, this test will pass even when it does not include the text("Some Text") because this sentence returns a unhandled promise.

// Should put `await` head
expect(waitForText("Some Text")).resolves.toBeTruthy();

[kobayang]

Sounds good to me.

[yamadayuki]

OK, I'll create PR to add the no-floating-promises rule as default.