Add interactive project initialization command

[nbeerbower]

The init command now creates a complete Hemlock project structure:

• src/main.hml with a basic hello world program
• package.json with scripts for start, test, and build
• .gitignore with Hemlock-specific patterns
• README.md with getting started instructions

Supports two modes:

• hpm init: Initialize project in current directory
• hpm init --name=my-project: Create new project directory