[proxy] Improve proxy typings

In its current version, the typings provided by the proxy package are relatively basic. They're smart enough to be able to provide a list of attributes that should be exposed per Three.js object instance, but there's a number of conversions we should be applying there for properties that can take multiple three-elements-specific inputs, like:

• [ ] Vector3 and Euler should be string | number[]
• [ ] Scalars should be string | number
• [ ] etc...?
• [ ] See Trinity for implementation

Also, further improvements include:

• [ ] Provide information which classes are actually being exposed by the proxy