VoxelSniper brushes

The plugin _VoxelSniper_ is sadly out of date and unmaintained. Many servers have used both _WorldEdit_ and _VoxelSniper_ as the core of their world sculpting tools, as there is incomplete overlap in the tool sets of both plugins.

I humbly request the addition of a few tools to the _WorldEdit_ tool set, to help alleviate this problem. There are a couple of _VoxelSniper_ functions in particular that I feel would greatly expand the utility of _WorldEdit_. (There are plenty more differences, but these two are frequently used in VS).

* The erode brush / tool. It has multiple modes in _VoxelSniper_ , but the one I've grown most fond of is the `melt` feature. Two tools linked to this (one with push and one with pull) allow landscapes to be massaged like soft clay.
* The `v` and `vr` commands. In modern _VoxelSniper_ , these select the block in line of sight (as set block and replace block, respectively). Whilst _WorldEdit_ has commands to choose the blocks to be set and/or masked, it isn't as smooth a process as just looking at it and typing a two letter command.

The license of _VoxelSniper_ is very permissive anyway, but rather than waste time with the whole kaboodle and all it's duplications of _WorldEdit_ features, it makes more sense to harness the best features and corral them within the main workhorse of the EngineHub stable, _WorldEdit_ itself.

This was briefly discussed on the EngineHub Discord, and @me4502 was prescient enough to know what I wanted before I even asked, so I made this issue as a persistent annoyance. 👯