Runtime generation of simple environment maps

## What problem does this solve or what need does it fill?

This proposes to kill several birds with one stone:
* Simplifying shaders by removing ambient lights and replacing them with simple 1x1 environment maps.
* Implementing hemispherical lights.

Creating environment maps for Bevy is a difficult process, particularly on Mac where you have to compile multiple C++ packages to get the tooling needed to do the conversion. Other game frameworks such as three.js provide a means to generate environment maps from an image at runtime.

Ambient lights can be emulated by a 1x1 environment map, which would mean that we could remove the shader code for ambient lights, thereby reducing the lines of code for our standard shaders.

## What solution would you like?

From [the discord](https://discord.com/channels/691052431525675048/743663924229963868/1331105565195112551):

```rust
foo.insert(EnvironmentMapLight::solid_color(SLATE_BLUE.into()))
foo.insert(EnvironmentMapLight::hemispherical_gradient(BLUE.into(), BROWN.into()))
```

The idea is to provide methods for generating simple environment maps from a set of parameters.

We should also offer a means to generate an environment map from an equirectangular image, but that is a more advanced project, and should probably be a separate issue.

For backwards compatibility, the `AmbientLight` can be transformed into an environment map light, either via the constructor or perhaps by a hook of some kind.

## What alternative(s) have you considered?

What I have done in the past is posted an image on Discord and had someone convert it to an environment map for me.

## Additional context

Supersedes: #12027