Renderer: Add hasInitialized().

[Mugen87]

Related issue: #29922 (comment)

Description

The PR adds hasInitialized() to Renderer so developers can check if the renderer is ready for usage.

[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	478.72 132.72	478.76 132.73	+42 B +8 B
WebGPU Nodes	478.18 132.6	478.23 132.61	+42 B +9 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	546.88 148.18	546.92 148.18	+42 B +8 B
WebGPU Nodes	502.76 137.89	502.8 137.9	+42 B +10 B