[563] Fix standalone comments inside complex blocks crashing Black (#4016)

Bracket depth is not an accurate indicator of standalone comment position inside more complex blocks because bracket depth can be virtual (in loops' and lambdas' parameter blocks) or from optional parens. Here we try to stop cumulating lines upon standalone comments in complex blocks, and try to make standalone comment processing more simple. The fundamental idea is, that if we have a standalone comment, it needs to go on its own line, so we always have to split.

This is not perfect, but at least a first step.