Add fallback spherical harmonics when dynamic environment lighting is unsupported

[ggetz]

Description

While the extensions needed for dynamic environment lighting are available in most systems, we were seeing downstream testing difficulties when using a WebGL stub that didn't support the required WebGL extensions.

This change:

• Is more explicit about requirements for dynamic updates to the environment map, adding a new function DynamicEnvironmentMapManager.isDynamicUpdateSupported to allow for checking.
• Provides a default diffuse lighting when dynamic updates are not supported. I picked an white ambient light with low intensity, as this is what we were already using in unit tests. This is accessible in DynamicEnvironmentMapManager.DEFAULT_SPHERICAL_HARMONIC_COEFFICIENTS for convenience.
• Cleans up and adds tests

Diffuse lighting when dynamic updates are disabled or not supported

Before: No diffuse light if dynamic updates were disabled

Issue number and link

N/A

Testing plan

1. CI should pass (CI runs with --webgl-stub)
2. Local tests should pass
3. For extra credit, checkout the effect when setting enabled to false, as in this Sandcastle

Author checklist

• [x] I have submitted a Contributor License Agreement
• [x] I have added my name to CONTRIBUTORS.md
• [x] I have updated CHANGES.md with a short summary of my change
• [x] I have added or updated unit tests to ensure consistent code coverage
• [x] I have updated the inline documentation, and included code examples where relevant
• [x] I have performed a self-review of my code

[github-actions[bot]]

Thank you for the pull request, @ggetz!

:white_check_mark: We can confirm we have a CLA on file for you.

[ggetz]

@jjspace This is ready for review!

[ggetz]

I was also seeing some extra spec failures on this branch locally that aren't happening in main. Not sure if it's directly related to this change or not

Good catch @jjspace, I must have forgotten to run all tests without --webgl-stub locally. These failures are expected. Fixed with some minor cleanup.