Support for custom trees in //forest

[sbrl]

It could be cool if we had some tooling to support the custom tree workflow.

Commands

//treeadd <name>

Could add the currently defined region as a custom tree, with a special indicator block determining the origin point that is placed directly underneath the centre of the tree.

//treelist

Could list the world's current list of custom trees.

//treeplace <name>

Could place the tree with the given name at pos1.

Naming scheme

A naming scheme would likely be required to avoid conflicts with the existing aliases. For example, we could require that all custom tree names begin with #.

Then, we might wan to support multiple variants for a given custom tree type, which the engine would randomly pick from when actually placing a forest, or the user could specify which variant they want. For example, #botwoak#3 would be the custom tree of type botwoak, variant 3. Hence, you could do this:

//forest aspen #botwoak

...which would place the default aspen saplings as normal and randomly selector from the registered variants of #botwoak, resulting in weighting like this:

+ 50% aspen
+ 50% #botwoak
    + 33% #botwoak#1
    + 33% #botwoak#2
    + 33% #botwoak#3

....or, you could specify a specific tree type like so:

//forest aspen #botwoak#3

...which would result in weightings like so:

+ 50% aspen
+ 50% #botwoak#3