Implement real streams or stream wrappers for result objects

[konard]

Summary

This PR implements issue #19 by replacing plain string properties in result objects with real Node.js stream wrappers, while maintaining full backward compatibility.

🔧 Implementation

Stream Wrappers:

• ReadableStreamWrapper for stdout and stderr
• WritableStreamWrapper for stdin
• Both extend Node.js native streams (Readable/Writable)
• Full stream functionality: pipe(), read(), write(), events, etc.

Core Changes:

• Modified createResult() to return stream wrappers instead of strings
• Updated virtual command processing to use the unified result creation
• Fixed stdin data handling for inherit mode
• Added proper capture mode support

Backward Compatibility:

• All string methods work unchanged: trim(), slice(), includes(), etc.
• String coercion: String(result.stdout) ✅
• JSON serialization: JSON.stringify(result.stdout) ✅
• Template literals: Output: ${result.stdout} ✅

💥 Breaking Changes

• typeof result.stdout changes from 'string' to 'object'
• This is intentional per the issue requirements for "real streams"
• Functionality remains compatible, only type inspection changes

🧪 Test Plan

• Comprehensive test suite for stream functionality
• Backward compatibility verification
• Both virtual and non-virtual commands tested
• Stream lifecycle and events tested
• Edge cases (empty streams, capture modes) covered

📖 Example Usage

const result = await $`echo "hello world"`;

// New stream functionality
result.stdout.on('data', chunk => console.log('Got:', chunk));
result.stdout.pipe(process.stdout);
result.stdin.write('input data');

// Existing string usage (still works)
console.log(result.stdout.trim());     // "hello world" 
console.log(result.stdout.length);     // 12
console.log(result.stdout.includes('hello')); // true

🏁 Status

✅ Ready for Review - Implementation complete with full test coverage

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🤖 Generated with Claude Code

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Resolves #19