Migrate browsergym, agentlab source code and nudging arena.

[hazel-ma2001]

Changes made in this PR

• Include the browsergym source code and adaptive modifications
• Include the agentlab source code by moving it ouside of the /src directory
• Include nudgingarena as a submodule
• Update .gitignore and README

Why are agentlab and browsergym kept local

• To add new benchmarks and tasks, it's necessary to adapt browsergym code
• To modify prompt templates and other agent settings, it's necessary to adapt agentlab code

Other changes inherited from my AgentLab fork

• Extend timeout from 10s to 50s for playwright ui-login and page loading because timeout error was often observed.
• Add error tracing and debug output at envs.py in browsergym