Allow Loaders to require onLoad parameter

[Methuselah96]

Why

Some Loaders check to see if onLoad is undefined before calling it, some just call it without an undefined check. With the changes in #570, it was impossible to define a Loader where the onLoad parameter was required since the base Loader class required it be optional.

This came up in practice because the loader types defined by three-stdlib have some required onLoad parameters that were causing type errors. After further investigation, this seemed like the best solution both for @types/three and three-stdlib.

What

This PR allows for either required or optional onLoad parameters by making the onLoad parameter required in the base Loader class and then overloading the load method in any derived classes that want to make onLoad optional.

Checklist

• Checked the target branch (current goes master, next goes dev)
• Added myself to contributors table
• Ready to be merged