Implement freeze() function for version freezing (issue #23)

[konard]

Summary

Implements experimental freeze() function that allows freezing dependency versions after first execution by modifying the source code using regex replacements.

Key Features

• Version Freezing: freeze('lodash') gets the latest version from npm and replaces the call with use('lodash@4.17.21')
• Cross-platform: Works with both ESM (.mjs) and CommonJS (.cjs) modules
• Syntax Preservation: Maintains await keyword when present
• Accurate File Detection: Uses stack trace analysis to find and modify the correct source file
• Error Handling: Comprehensive error handling for various edge cases

Usage Examples

ESM (ES Modules):

import { use, freeze } from 'use-m';

// Before first run:
const _ = await freeze('lodash');

// After first run, file is automatically modified to:
const _ = await use('lodash@4.17.21');

CommonJS:

const { use, freeze } = require('use-m');

(async () => {
  // Before first run:
  const chalk = await freeze('chalk');
  
  // After first run, file is automatically modified to:
  const chalk = await use('chalk@5.6.2');
})();

Implementation Details

• Uses npm view <package> version to fetch latest version
• Employs regex pattern matching to find freeze() calls in source files
• Captures stack traces at function call time (before async operations) for accurate caller context
• Handles both file:// URLs (ESM) and absolute paths (CommonJS) from stack traces
• Preserves existing code formatting and await keywords
• Supports all package naming conventions including scoped packages

Testing

• ✅ ESM freeze functionality with lodash and simple-git
• ✅ CommonJS freeze functionality with chalk
• ✅ Existing test suite passes (Node.js tests)
• ✅ No breaking changes to existing functionality

Closes #23

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