Allow filling the edges of a selection

### The Problem

Currently, the `//faces` command exists to fill the six faces of a cuboid region. However, there is no way to specify to only fill the outer edges of this cuboid region.

### A Solution

A complement command could be introduced with the syntax `//edges `. For a cuboid region, this would fill its twelve edges. The behavior for other regions is more difficult to define, but the main purpose of this command would be to use it with cuboid regions.

### Alternatives

The closest solution I know of is using `//faces` then manually changing the difference between faces and edges back to air.