@std/xml: support a "lax-mode" to allow parsing html

XML is basically a stricter version of HTML, so most of the work is already done and it shouldn't require too much refactoring

Parsing HTML is a quite common use case (like when scraping websites, parsing web pages, etc) but since the data is external we can't expect them to be exact html

Parsing html without requiring a full virtual dom library (like jsdom / happy-dom /linked-dom) would be quite nice for general use case, but also for tests validations when testing http documents serves

There are already precedent of having a more leniant parsing in other libs
- quick-xml (Rust): check_end_names(false)
- libxml2 XML_PARSE_RECOVER / lxml recover=True
- htmlparser2 and fast-xml-parser in the JS ecosystem
- [`jsr:@libs/xml`](https://jsr.io/@libs/xml) (which i maintain and like to rely on `@std/xml` now rather than a compiled wasm version of quick-xml)

What would be required:
- Supporting unquoted attribute values: `` (value ends at whitespace, `>`, or `/`)
- Valueless attributes `` (same as `disabled=""`)
- Unclosed / mismatched elements: `` auto-closes intervening open elements (stray closing tags are ignored, elements still open at EOF are auto-closed instead of throwing)
- Duplicate attributes: first occurrence wins instead of throwing.

Btw the sync parse does not offer any way to recover the xml processing instructions nor the doctype atm

cc @tomas-zijdemans

Start by reading the @std/xml parser entry point and tracing its current strict handling of attributes, closing tags, and duplicate attributes. Done means a lax mode supports the four listed recovery behaviors, while preserving strict parsing when lax mode is not enabled.

Written by the indexing model from the issue text.