Add custom task system for BrowserGym environment

[atchudhansg]

Add Custom Task System for BrowserGym

Summary

Adds a custom task framework that allows users to create domain-specific BrowserGym tasks without depending on official benchmark packages.

Changes

• Custom task framework: Base classes, models, and registry system for creating tasks
• Example tasks: Copy-paste (single-tab) and copy-paste-multitab implementations
• Documentation: Comprehensive guide in README with examples and best practices
• Example script: browsergym_custom_example.py demonstrating usage
• Test coverage: Basic tests for custom task system

Key Features

• No BrowserGym package dependencies required
• Direct Playwright browser control
• Simple registration system
• Rapid prototyping for domain-specific tasks
• Follows MiniWoB design philosophy (minimal HTML, functional UI)

Usage

env = BrowserGymEnv(environment={
    "BROWSERGYM_BENCHMARK": "custom",
    "BROWSERGYM_TASK_NAME": "copy-paste"
})

Files Changed

• custom - Custom task system implementation
• README.md - Updated with custom tasks documentation
• browsergym_custom_example.py - Usage example
• .gitignore - Added myenv to exclude virtual environments