feat: add Mill build system and Google Java Format

[taciturnaxolotl]

Summary

This PR introduces Mill as a modern alternative build system to Gradle, along with Google Java Format for consistent code styling.

What is Mill?

Mill is a modern build tool that offers fast incremental compilation, simple configuration, and built-in caching. It works identically across Windows, macOS, and Linux.

Fancy features

• Everything works with mill! - All WPILib 2025 and vendor dependencies work so far; more testing with actually deploying is needed though
• Google Java Format - Automatic code formatting with git hooks that should be consistent across all machines
• GitHub CI - Automated formatting checks and compilation to catch stuff earlier

Installation

# Install Mill
curl -L https://github.com/com-lihaoyi/mill/releases/latest/download/mill > mill && chmod +x mill

# Or: brew install mill (macOS) / choco install mill (Windows)

# Setup project
mill setupGitHooks  # Install pre-commit formatting
mill compile        # Build robot code
mill deploy         # Deploy to RoboRIO

Gradle still works - this is additive. We can try Mill while keeping Gradle as backup.