'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)

etc/schema/dotnet-slnx.rnc

@@ -0,0 +1,74 @@
+default namespace = ""
+namespace a = "http://relaxng.org/ns/compatibility/annotations/1.0"
+namespace ns_1 = "http://relaxng.org/ns/compatibility/annotations/1.0"
+namespace rng = "http://relaxng.org/ns/structure/1.0"
+
+start |= starting_Solution
+starting_Solution =
+  element Solution {
+    (element Configurations { Configurations }?
+     | element Project { Project }*
+     | element Folder { Folder }*
+     | PropertiesGroup*)*,
+    attribute Description { xsd:string }?,
+    attribute Version { xsd:string }?
+  }
+Configurations =
+  (element BuildType {
+     attribute Name { xsd:string }
+   }*
+   | element Platform {
+       attribute Name { xsd:string }
+     }*
+   | element ProjectType { ProjectType }*)*
+ProjectType =
+  (ConfigurationRulesGroup*)*,
+  attribute TypeId { xsd:string }?,
+  attribute Name { xsd:string }?,
+  attribute Extension { xsd:string }?,
+  attribute BasedOn { xsd:string }?,
+  [ ns_1:defaultValue = "true" ] attribute IsBuildable { xsd:boolean }?,
+  [ ns_1:defaultValue = "true" ]
+  attribute SupportsPlatform { xsd:boolean }?
+Folder =
+  (element File {
+     attribute Path { xsd:string }
+   }*
+   | element Project { Project }*
+   | PropertiesGroup*)*,
+  attribute Name { xsd:string }
+Project =
+  (element BuildDependency {
+     attribute Project { text }
+   }*
+   | ConfigurationRulesGroup*
+   | PropertiesGroup*)*,
+  attribute Path { xsd:string },
+  attribute Type { xsd:string }?,
+  attribute DisplayName { xsd:string }?
+PropertiesGroup =
+  element Properties {
+    element Property {
+      attribute Name { xsd:string },
+      attribute Value { xsd:string }?
+    }*,
+    attribute Name { text },
+    attribute Scope { text }?
+  }
+ConfigurationRulesGroup =
+  element BuildType {
+    attribute Solution { xsd:string }?,
+    attribute Project { xsd:string }?
+  }+
+  | element Platform {
+      attribute Solution { xsd:string }?,
+      attribute Project { xsd:string }
+    }+
+  | element Build {
+      attribute Solution { xsd:string }?,
+      attribute Project { xsd:string }?
+    }+
+  | element Deploy {
+      attribute Solution { xsd:string }?,
+      attribute Project { xsd:string }?
+    }+

etc/schema/schemas.xml

@@ -93,4 +93,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>. -->
   <documentElement localName="Project" typeId="MSBuild"/>
   <typeId id="MSBuild" uri="msbuild.rnc" />
 
+  <uri pattern="*.slnx" typeId="Dotnet Slnx" />
+  <typeId id="Dotnet Slnx" uri="dotnet-slnx.rnc" />
+
 </locatingRules>

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)