'nxml-mode': add schema for .NET SLNX files.

SLNX is a new XML-based file-format for .NET based solutions, replacing
the older proprietary SLN file-format.

Unlike SLN-files, it is merge-friendly and expected to become the new
de-facto standard for working with .NET projects.  This commit adds
support for SLNX-schema to 'nxml-mode'.

The schme provided has been synthesized using the official XSD-schema
definition:
https://github.com/microsoft/vs-solutionpersistence/blob/main/src/Microsoft.VisualStudio.SolutionPersistence/Serializer/Xml/Slnx.xsd

This schema was then converted to RNG using XSDtoRNG.xsl:
https://github.com/epiasini/XSDtoRNG

The RNG schema was then converted to RNC using the trang CLI-tool:
https://relaxng.org/jclark/trang.html

* etc/schema/dotnet-slnx.rnc: New file.
* etc/schema/schemas.xml: Add Slnx schema.

* lisp/files.el (auto-mode-alist): Add association for SLNX files.
(Bug#79379)

lisp/files.el

@@ -3158,6 +3158,7 @@ ARC\\|ZIP\\|LZH\\|LHA\\|ZOO\\|[JEW]AR\\|XPI\\|RAR\\|CBR\\|7Z\\|SQUASHFS\\)\\'" .
     ("\\.oak\\'" . scheme-mode)
     ("\\.sgml?\\'" . sgml-mode)
     ("\\.x[ms]l\\'" . xml-mode)
+    ("\\.slnx\\'" . xml-mode)
     ("\\.dbk\\'" . xml-mode)
     ("\\.dtd\\'" . sgml-mode)
     ("\\.ds\\(ss\\)?l\\'" . dsssl-mode)