Migrate to Pure ES Modules and Drop CommonJS Support

## Background
Currently, Hasty Server supports both CommonJS (`require()`) and ES Modules (`import`) module systems. While this dual support is helpful for compatibility, maintaining both systems increases complexity and goes against the Node.js ecosystem's direction towards ES Modules as the standard.

## Current State
- The project currently supports both module systems
- Package can be imported using both `require()` and `import`
- Examples in documentation show both syntaxes
- No clear preference is indicated in the documentation

## Proposed Changes
1. **Package Structure Updates:**
- Convert all source files to use `.mjs` extension
- Update `package.json` to include `"type": "module"`
- Remove CommonJS-specific code paths

2. **Documentation Updates:**
- Update all code examples to use ES Module syntax
- Add migration guide for users moving from CommonJS
- Update installation and usage instructions

3. **Implementation Details:**
- Replace `module.exports` with `export default`
- Convert any `require()` calls to `import` statements
- Update test files to use ES Module syntax
- Implement proper error messages for CommonJS usage attempts

## Benefits
- Aligned with Node.js future direction
- Simplified codebase maintenance
- Better tree-shaking support
- Cleaner syntax and improved developer experience
- Better compatibility with modern JavaScript tools and bundlers

## Breaking Changes
This will be a breaking change requiring a major version bump:
- Users will need to update their import statements
- Projects using CommonJS will need to migrate
- Minimum Node.js version requirement may need to be updated

## Implementation Plan
1. Create new branch for ES Module migration
2. Update source files one at a time
3. Update tests to ensure functionality remains intact
4. Update documentation and examples
5. Create comprehensive migration guide
6. Release as major version update (e.g., v1.0.0)

## Questions to Consider
- What should be the minimum supported Node.js version?
- Should we provide a transition period with both systems?
- How can we best support users during migration?

Would appreciate feedback on this proposal, especially regarding the implementation plan and transition strategy.