feat: make SVG content conformance checking informative-only

[rdeltour]

EPUB 3.3 no longer requires that SVG content conforms to SVG content model requirements,
only that they are well-formed, that ID are uniques, and that they respect some additional
EPUB-specific requirements.

This PR:
* introduces a new permissive RelaxNG schema for SVG, checking only the EPUB-specific
requirements on the title and foreignObject content model
see w3c/epub-specs#1323
* removes checks on the value of the requiredExtensions attribute of foreignObject
see w3c/epub-specs#1087
* adapts the main XHTML to SVG schema driver to the new permissive SVG schema
* adds various tests for EPUB-specific requirements

The PR also introduces two new codes, RSC-024 and RSC-025, to report informative schema checks.

An XMLValidator instance can now be instantiated with a boolean flag to tell if the unerlying schema is normative or not.

If a schema is not normative, the XMLParser uses an informative-only ErrorHandler, that reports warnings as RSC-024 (USAGE), and errors as RSC-025 (USAGE).
Fatal errors are still reported as RSC-016 (FATAL).

SVG schema-based content conformance checking is made informative only:
* new NVDL and RelaxNG schema drivers will only check SVG content, and ignore other
namespaces.
* these informative schemas are enabled for both SVG and XHTML Content Documents
* SVG content conformance, as before, is using RelaxNG schemas from the Nu HTML Checker.

[mattgarrish]

In general this looks great, but why are the following messages being emitted for every SVG document:

USAGE(HTM-010): .\30\sous-le-vent_svg-in-spine.epub/EPUB/Content/pageNum-1.svg(3,186): Namespace uri "http://www.w3.org/2000/svg" was found.

Looking at the source, I believe it comes from this:

   if (!isExpectedNamespace(uri))
    {
      report.message(MessageId.HTM_010,  location, uri);
    }

But wouldn't the svg namespace be expected on an svg document?

It seems to report every namespace declared, as I also get the same message for the xlink namespace when xmlns:xlink="" is declared.

[rdeltour]

That's no longer the case in v5.0.0-beta-3, it will be removed in 2c429d0 (big URL refactoring)