✨ feat(serialize): add XML/XHTML output syntax

[gaborbernat]

turbohtml serialized HTML syntax only, so a tree bound for an XML toolchain (an XSLT or XPath 2.0 pipeline, an XMLSerializer consumer, anything that rejects HTML's unclosed tags) had to be round-tripped through a second library. This adds an XML/XHTML serialization mode, the equivalent of lxml's tostring(method="xml"), so serialize, encode, and serialize_iter can emit well-formed XML from the same parsed tree.

Html gained an xml field. With xml=True the WHATWG XML serialization rules apply: every empty element self-closes (<br/>, <div/>), a foreign SVG or MathML subtree carries the namespace declaration that makes it well-formed (xmlns on the subtree root, xmlns:xlink on any element bearing an xlink: attribute), and text and attribute values follow the XML escaping rules (&, <, > in text, plus " and the whitespace characters in attributes, with a no-break space left literal since XML predefines no  ). The HTML void-element and raw-text special casing does not apply, so a <script> body is escaped like any other text.

The serializer keeps its single tree walk and branches on the flag at the four points where the HTML and XML algorithms diverge, so the HTML fast path stays intact. 🚀 The flag composes with sort_attributes and an Indent layout; a Minify layout stays HTML, since its optional-tag and unquoted-attribute rules are HTML-parser rules.

The docs/migration/lxml.rst guide maps tostring(method="xml"|"xhtml") onto serialize(Html(xml=True)), and a serialize-xml benchmark op compares turbohtml against lxml's method="xml" head to head.

closes #535

[codspeed-hq]

Merging this PR will not alter performance

✅ 66 untouched benchmarks
🆕 1 new benchmark
⏩ 18 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	test_feature[serialize-xml]	N/A	2.9 ms	N/A

---

_{Comparing gaborbernat:feat/535-xml-output (478d12f) with main (e9032d7)}

Footnotes

1. 18 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩