New "setcorners" command to set border based on corner coordinates: /wb [world] setcorners <x1> <z1> <x2> <z2>
This is useful now that border shapes can be rectangular/elliptical with different X and Z radius values.

Also various cleanup of new rectangular/elliptical support. Commands can now accept a single radius value (as it used to) or separate X and Z. Radius is displayed as single number if X and Z are equal. "shape" and "wshape" commands accept round|square as before for backwards compatibility and simplicity, alongside the new elliptical|rectangular (which work the same). Updated config loader to support loading older configs with single radius value specified. Added various other methods for backwards compatibility.