fix(no-async-in-computed-properties): Allow async in nested scopes

[michalsnik]

Fixes #725

[armano2]

This is not going to be perfect is still going to warn about:

foo() {
 const foo = {
   async bar() => await baz()
 }
 return foo
}

@michalsnik what do you think about using Code Path Analysis to determine if function is reachable?

on other hand there if we implement in this way we are not going to catch...

foo() {
 const foo = {
   async bar() => await baz()
 }
 return foo.bar()
}

[armano2]

Its looks ok, it's good improvement.

[michalsnik]

Yeah, it's a pet peeve. We'll never be 100% covered, but at least we can minimize most common false positives. Let's iterate and see whether someone will happen to struggle with the other case.