feat(parser): add YAHTML AST parser API

[han4wluc]

Summary

• add parseElementKey API for selector/attribute parsing with offsets
• add parseYahtmlAst API to build structured YAHTML AST nodes (root, element, text, rawHtml, doctype, fragment)
• keep convertToHtml behavior unchanged while exposing parser APIs from package entrypoints
• split self-closing tags to shared src/constants.js
• move parser tests into Puty YAML specs under spec/parser.spec.yaml for parser-grade coverage
• bump package version from 0.0.4 to 0.0.5

Validation

• bun run test (passes: 130/130)