[enhancement] more useful #surface command

In the F3 debug menu, in the underlying sections CH and SH we can see the client and server side heightmap information. In accordance with these values, we can refine the #surface command to find a better target block that is actually at the surface, rather than a fixed y=64 value or a manual block-by-block check. y=64 or alternatives could still be a fallback in case both of these heightmaps are ill-formed.