fix: preserve XHTML content order in RSS feeds

[MossabArektout]

This PR Fixes #275

Summary

Fixes issue where XHTML content elements were parsed out of order, causing concatenated text without proper separation between elements.

Problem

When parsing RSS feeds with XHTML content, elements like <strong> tags and <small> tags were being concatenated without proper separation, resulting in malformed output like:

Solution

• Added buildOrderedContent() function in lib/utils.js to preserve element sequence
• Integrated ordered XML parser using xml2js preserveChildrenOrder option
• Updated both Atom and RSS parsers to use ordered content when available
• Added comprehensive test case to verify the fix

Files Changed

• lib/utils.js: Added buildOrderedContent() function
• lib/parser.js: Added ordered XML parser and integration
• test/parser.js: Added test case for XHTML content order preservation
• test/input/xhtml-content-order.rss: Test fixture
• test/output/xhtml-content-order.json: Expected test output

Testing

• All existing tests pass
• New test case "should preserve XHTML content order" passes
• Content ordering is maintained properly in both Atom and RSS feeds