BREAKING(xml): clean up public API ahead of stabilization

[tomas-zijdemans]

Summary

Cleanup of the @std/xml public API in preparation for a future
stabilization proposal. The goal is to remove surface that should not be
stabilized and fix documentation that did not match actual behavior, so
the API that remains is the API we would commit to at 1.0.0.

Breaking changes

• Remove unused exported types. The XmlEvent union and its event
  interfaces (XmlStartElementEvent, XmlEndElementEvent, XmlTextEvent,
  XmlCDataEvent, XmlCommentEvent, XmlProcessingInstructionEvent) and
  XmlAttribute were leftovers from a pre-callback event-object design and
  appeared in no public function signature. XmlTokenCallbacks is the
  tokenizer's internal contract and has moved to _tokenizer.ts.
• Rename XmlDeclarationEvent to XmlDeclaration. It survives only as the
  type of XmlDocument.declaration, so the Event suffix was a misnomer.
• XmlSyntaxError no longer appends at line 0, column 0 to messages
  when position tracking is disabled. line: 0 is documented as the
  sentinel for "position unknown".

Documentation fixes

• XmlPosition.offset and XmlSyntaxError.offset are UTF-16 code-unit
  offsets into the input string, not byte offsets.
• XmlElement.attributes is keyed by the raw qualified attribute name
  (e.g. xlink:href, xmlns:ns), not the local name.
• XmlEventCallbacks.onDoctype now documents that it only fires when
  disallowDoctype: false is set; under the default, a DOCTYPE throws.

Tests

• Public-level parseXmlStream() DOCTYPE tests: onDoctype receives name,
  public ID, and system ID; the default rejects DOCTYPE without invoking
  the callback (previously only covered at the internal tokenizer level).
• Tests for the position-suffix behavior of XmlSyntaxError.

Test plan

• deno task ok passes locally

---

This PR was developed with the assistance of an AI agent in Cursor; all
changes were reviewed and verified by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (0e5b1f2) to head (e064d23).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7206   +/-   ##
=======================================
  Coverage   94.83%   94.84%           
=======================================
  Files         617      618    +1     
  Lines       51674    51680    +6     
  Branches     9350     9351    +1     
=======================================
+ Hits        49007    49015    +8     
  Misses       2121     2121           
+ Partials      546      544    -2     

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