Calculate the modified property based on cached values

The Lua API provides access to several attribute and skill properties. Changing these properties results in a delayed stat change on the actor, but accessing the property again will return the modified value immediately.

The exception to this is the modified property, which is a read only value computed from the other properties. This MR makes the property return a computed value based on the potentially modified other properties of that skill or attribute.

https://discord.com/channels/260439894298460160/854806553310920714/995967278644482069