Add EnvironmentMapLight::with_intensity builder method

[tylercritchlow]

closes #23911

Objective

• Add a way to set the intensity of an EnvironmentMapLight.

Solution

• Create a builder-style with_intensity method so that you can set intensity easily while constructing.

EnvironmentMapLight::hemispherical_gradient(assets, sky, horizon, ground)
    .with_intensity(250.0)

Testing

• This is a small convenience method with no behavioral change to existing code, so no new tests were added.
• CI