Improve tool calling

• Update ReAct agent system prompt to return Action in JSON format
• Allow Tool to accept multiple arguments
• Extract logic potential common for different engines into Regent::Engine::Base
• Update LLM interface:
  • Add strict_mode optional argument
• Update Agent interface:
  • execute method renamed to run
  • llm argument renamed to model
  • Add engine optional argument for future use with different engine

Full Changelog: v0.3.1...v0.3.2