MathNode: Add `rand()`.

[Mugen87]

Related issue: -

Description

This ports the rand() GLSL helper function from commons.glsl to TSL and adds it to MathNode.

rand() is used multiple times in the addons e.g. in FilmPass that I want to port next.

[github-actions[bot]]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
682.2 kB (169 kB)	682.2 kB (169 kB)	+0 B

🌳 Bundle size after tree-shaking

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

Filesize dev	Filesize PR	Diff
459.4 kB (110.9 kB)	459.4 kB (110.9 kB)	+0 B

[sunag]

/cc @brunosimon