WIP: XML API: Introduce XML Processor.

[dmsnell]

Introduces one more XML Processor.

• It ignores DTDs, merely parsing their syntax and ignoring their semantics.
• It makes no network calls while parsing and loads no external documents.
• The only entity expansion performed is for the five built-in entities.
• It requires UTF-8 encoding of the input document, but passes through invalid byte sequences.
• Rejects fatal errors in parsing, such as if the document is not well-formed^†.
• Otherwise is a recoverable parser, taking liberties to let garbage remain garbage.

This parser exposes errors and fatal errors to calling code so that individual applications can determine how to perform recovery or reject the document.

See also adamziel#44
See also adamziel#43
See also https://github.com/WordPress/wordpress-develop/compare/trunk...sirreal:wordpress-develop:xml-processor-continue?expand=1

[github-actions]

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