linter: add finally return check for unreachable checker

[i582]

Report the use of return in finally when
there are other return, throw in the try-catch.

The problem is they will be unreachable.

For example:

function f(): int {
  try {
    throwException();
  } catch (Exception $_) {
    return 2; // unreachable
  } finally {
    return 1;
  }
}

f(); // always return 1

Report the use of die in catch when
there are finally block.

The problem is finally will be unreachable.

function f(): int {
    try {
        throwException();
    } catch (RuntimeException $_) {
        return 2;
    } catch (Exception $_) {
        die();
    } finally { // block finally is unreachable (because 2 catch block contains a exit/die)
        return 1;
    }
}

#332