AngelScript: 3D math functions

# 3D math functions

Path: /sdk/add_on/scriptmath3d/

This add-on registers some value types and functions that permit the scripts to perform 3D mathematical operations. Use `RegisterScriptMath3D(asIScriptEngine*)` to perform the registration.

Currently the only thing registered is the `vector3` type, representing a 3D vector, with basic math operators, such as add, subtract, scalar multiply, equality comparison, etc.

This add-on serves mostly as a sample on how to register a value type. Application developers will most likely want to register their own math library rather than use this add-on as-is.