Line2NodeMaterial: Rename useColor property to vertexColors

[sunag]

Fixes #32668

Description

It appears that there are duplicate names and the property is not following the naming convention adopted in the other classes.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.36 84.47	355.36 84.47	+0 B +0 B
WebGPU	621.49 172.57	621.5 172.57	+8 B +1 B
WebGPU Nodes	620.1 172.32	620.11 172.33	+8 B +1 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	487.23 119.23	487.23 119.23	+0 B +0 B
WebGPU	692.41 187.81	692.41 187.81	+0 B +0 B
WebGPU Nodes	642.21 175.03	642.21 175.03	+0 B +0 B