Add support for Google Gemini API

[sladwig]

Feature: Add support for Google Gemini API

This issue is to add support for Google's Gemini AI models to MyCoder.

Implementation details:

• Add a new GeminiProvider class that implements the LLMProvider interface
• Install @google/generative-ai package as a dependency
• Register the provider in the provider registry
• Update configuration examples and documentation
• Add appropriate token context window sizes for Gemini models

Tasks:

• Create gemini.ts provider implementation
• Add Gemini to provider registry
• Update README.md to mention Gemini support
• Create documentation for the Gemini provider
• Update configuration examples

Notes:

• Gemini API differs from OpenAI and Anthropic in how it handles messages and function calling
• Token usage reporting is estimated as Gemini doesn't provide exact token counts