Add a rule prefer_guard_clause for reversing nested if statements

[velvitoff]

No description provided.

[yurii-prykhodko-solid]

The idea seems to mostly work, but I think we lack a clear definition of what we're trying to do here.

I'd look to these cases:

1. when if block is spanning the entire function body:

void fn() {
  if (true) {
  ...
  }
}
// ->>
void fn() {
  if (false) return;
  ...
}

2. when if-else is spanning the entire function body

[yurii-prykhodko-solid]

This looks like a great start for this lint!
We can expand on it in next iterations.
For now, just need to fixup a few minor issues, and clarify a few pieces.

[yurii-prykhodko-solid]

LGTM!

[illia-romanenko]

LGTM - but missing changelog, I'll add it myself.