PMREMGenerator: Add fromSceneAsync()

[sunag]

Related issue: #29942

Description

Based on the PR we can see that the webgl_animation_keyframes example emits a lot of error messages that may not make much sense to the user and it doesn't load the IBL properly.

This is because PMREMGenerator is being used without the WebGPU Backend being initialized. This PR aims to improve the error messages by guiding the user to replace functions like pmremGenerator.fromScene() with pmremGenerator.fromSceneAsync().

After this PR.

The user can fix it using:

scene.environment = ( await pmremGenerator.fromSceneAsync( new RoomEnvironment(), 0.04 ) ).texture;

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.13 79	339.13 79	+0 B +0 B
WebGPU	483.69 134.01	484.71 134.2	+1.02 kB +191 B
WebGPU Nodes	483.16 133.9	484.18 134.1	+1.02 kB +192 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	464.59 111.96	464.59 111.96	+0 B +0 B
WebGPU	550.99 149.23	552.01 149.42	+1.02 kB +194 B
WebGPU Nodes	506.87 138.94	507.89 139.13	+1.02 kB +192 B