parser: Handle self-closing elements and void elements

[paulsmith]

• Add selfClosing field to nodeElement struct to track self-closing tags
• Update match function to accept multiple token types
• Check for self-closing tags in parseElement and set selfClosing field
• Return early for void elements in parseElement
• Refactor parseChildren to handle both start and self-closing tags
• Add isVoidElement function to check if a tag is a void element
• Update tests to cover self-closing and void elements

🌟 These changes improve the HTML parser to properly handle self-closing elements and void elements according to the HTML spec.

Fixes #126