Plane: Make second set parameter optional.

[Psychpsyo]

Calling Plane.set() without passing a value for constant results in a plane with a constant of undefined.
The docs already say that this parameter should default to 0.

This makes it actually do that.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
674.7 kB (167.2 kB)	674.7 kB (167.2 kB)	+2 B

🌳 Bundle size after tree-shaking

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

Filesize dev	Filesize PR	Diff
454.1 kB (109.6 kB)	454.1 kB (109.6 kB)	+2 B

[Mugen87]

I'm not sure about this change. A plane definition consists of a normal and constant. It seems inconsistent to just have a default parameter for one of them.

I would rather suggest to updated the docs and remove the comment about the default value (this is a clear mistake). Both set() parameters should be mandatory.