Implement package exports and subpath imports handling

[Copilot]

This PR implements smart handling of package exports and subpath imports to distinguish between legacy packages that need .js extensions and modern packages with exports field that should remain unchanged.

Problem

The current implementation adds .js extensions to all node_modules imports, but this doesn't work correctly for modern packages that use the exports field in their package.json. This causes issues where:

• Legacy packages (like lodash) need .js extensions: 'lodash/omit' → 'lodash/omit.js'
• Modern packages (like firebase-functions) should remain unchanged: 'firebase-functions/v1/https' (already works correctly in ESM)

Solution

Added intelligent package detection that checks if a node_modules package has an exports field:

// Before: All packages got .js extensions
import omit from 'lodash/omit';           // → 'lodash/omit.js' (correct)
import {HttpsError} from 'firebase-functions/v1/https'; // → 'firebase-functions/v1/https.js' (incorrect!)

// After: Smart detection based on package.json exports field
import omit from 'lodash/omit';           // → 'lodash/omit.js' (legacy package)
import {HttpsError} from 'firebase-functions/v1/https'; // → unchanged (modern package)

Key Changes

1. Added hasPackageExports() utility in PathUtil.ts that safely reads and parses package.json files to detect the exports field
2. Enhanced replaceModulePath.ts to check package exports before adding file extensions
3. Added support for scoped packages with proper parsing of package names like @types/node
4. Comprehensive test coverage including mock packages for both scenarios

Testing

• All existing tests continue to pass
• New test fixture with realistic mock packages (lodash as legacy, firebase-functions as modern)
• Manual validation confirms correct behavior for both package types
• Added unit tests for new utility functions

This change maintains full backward compatibility while fixing the issue with modern packages that use the exports field.

Fixes #129.

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