This adds support for computer selectors, in the style of entity
selectors. The long-term goal here is to replace our existing ad-hoc
selectors. However, to aid migration, we currently support both - the
previous one will most likely be removed in MC 1.21.
Computer selectors take the form @c[<key>=<value>,...]. Currently we
support filtering by id, instance id, label, family (as before) and
distance from the player (new!). The code also supports computers within
a bounding box, but there's no parsing support for that yet.
This commit also (finally) documents the /computercraft command. Well,
sort of - it's definitely not my best word, but I couldn't find better
words.
