Scripts: Add changelog generator.

[mrdoob]

Description

Adds utils/changelog.js - a script to automatically generate release changelogs.

Features:

• Groups commits by category based on changed files and title prefixes
• Separates core changes, addons, docs, editor, examples, etc.
• Fetches PR info via GitHub CLI for accurate author attribution
• Handles co-authors from commit messages
• Skips build commits, dependency updates, and reverted commits
• Outputs markdown ready for GitHub releases

Usage:

node utils/changelog.js

Requires GitHub CLI to be installed.