Create a BlockPositionCommandElement

[ryantheleach]

#1917 Shows that some confusion can arise using the existing Vector command elements.

I propose splitting the generic Vector command element to not accept #target and only accept raw vectors and instead introduce 2 alternatives.

BlockPositionCommandElement
and
ExactPositionCommandElement

BlockPositionCommandElement could potentially have additional parsing options.

e.g. if the player capable of flight? Should the nearest solid floor be used? Whether a block, or a space ontop, infront, of a block should be selected etc.

This would be a utility on top of the ray tracing to aid auto complete.

ExactPositionCommandElement would act like the current Vector3 command element.