feat: replace cosmiconfig with c12 for configuration management

[bhouston]

Replace cosmiconfig with c12 for configuration management

This PR replaces cosmiconfig with c12 for better TypeScript support and additional configuration features, as requested in issue #260.

Changes

• Replaced cosmiconfig with c12 for configuration loading
• Added configuration watching capability via watchConfigForChanges function
• Removed dependency on cosmiconfig
• Maintained backward compatibility with existing configuration files
• Fixed type issues when working with c12's configuration types

Benefits of c12 over cosmiconfig

1. TypeScript Support: c12 has built-in TypeScript support without requiring custom loaders
2. More Configuration Formats: Supports JSONC, JSON5, TOML in addition to standard formats
3. Configuration Directory: Supports a .config/ directory for organizing configuration files
4. Environment-specific Configurations: Allows for environment-specific configuration files
5. Extended Configurations: Supports extending configurations from other files or packages
6. Configuration Watching: Built-in support for watching configuration files for changes
7. Dotenv Support: Built-in support for loading environment variables from .env files
8. Active Development: Part of the UnJS ecosystem, actively maintained

Testing

• All tests pass with the new implementation
• The API remains compatible with existing code

Closes #260