Add missing boolean readonly props to lights and materials

[michealparks]

Why

Currently it's difficult to type-narrow any class that does not have a typedef including an is* or type property that specifies a string constant as a value.

What

This PR starts to address this problem by adding missing boolean readonly props to Lights and Materials.

Checklist

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

[Methuselah96]

Thanks!