WebXRManager: Further improvements to WebXR layers feature testing.

[toji]

Addresses concerns brought up in #30112

Description

Further improves the feature detection for Layers in WebXR so that systems that only allow a single projection layer (which is valid according to the spec) can still take the layers path in Three for most content. Thanks for highlighting the issue, @cabanier! Please let me know what you think about this approach?

Also attempts to address the issue with Pico's browser that was brought up in #30112. I do not have Pico hardware, so I can't test, but my theory is that because the code was previously checking for session but my patch added a check for session.enabledFeatures before that if it was hit this code path prior to a session being created it would display errors in the console. If anyone has access to the Pico browser and could verify that hypothesis, I'd appreciate it!

Pushed as a draft initially because I haven't had a chance to test the patch against the Meta browser just yet. I'll take off the draft label once I validate that it works there.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.51 79.09	339.52 79.1	+11 B +7 B
WebGPU	490.96 136.34	490.96 136.34	+0 B +0 B
WebGPU Nodes	490.43 136.23	490.43 136.23	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.35 112.14	465.36 112.15	+11 B +9 B
WebGPU	560.26 151.75	560.26 151.75	+0 B +0 B
WebGPU Nodes	516.35 141.53	516.35 141.53	+0 B +0 B