Loaders: Make return types in load() consistent.

[Mugen87]

Fixed #10712.

Description

The PR makes the return types of load() in FileLoader and ImageBitmapLoader more consistent which solves the long-standing issue #10712. Both methods adhere now to the signature of Loader.load().

Because FileLoader does not rely on XMLHttpRequest anymore and abort() is implement, it's was much easier to implement a fix compared to nine years ago. The return value of load() is now independent of the cache state. This avoids an unreliable return value that could not be properly used in production anyway.

The PR also removes two return statements in ImageBitmapLoader which had no effect (they were located inside Promises and nowhere consumed).

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	361.21 85.78	361.21 85.78	+0 B +0 B
WebGPU	637.48 177.06	637.48 177.06	+0 B +0 B
WebGPU Nodes	635.59 176.76	635.59 176.76	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	493.58 120.42	493.58 120.42	+0 B +0 B
WebGPU	709.22 191.88	709.22 191.88	+0 B +0 B
WebGPU Nodes	658.44 179.17	658.44 179.17	+0 B +0 B