Add support for typing accessors/modifiers (getters/setters)

[Benjamin-Dobell]

Lua's metatable and userdata functionality both provide mechanisms for a "property" accessor on a table/userdata to return a value of one type, whilst a different type may be accepted for assignment. We should provide a mechanism to type such "properties".

Additionally, some "fields" are read or write-only; perhaps these could be typed by simply marking a setter or getter type as void?