Add .slnx to XML extensions (#7084)

* Recognize .slnx as XML

* Add a sample .slnx

* Delete samples/Microsoft Visual Studio Solution/Robots.slnx

* Create Robots.slnx

* Add support for <Project and <Solution tag for auto-recognition of xml

* revert

* Update test_strategies.rb

Author: kasperk81

lib/linguist/languages.yml

@@ -8346,6 +8346,7 @@ XML:
   - ".scxml"
   - ".sfproj"
   - ".shproj"
+  - ".slnx"
   - ".srdf"
   - ".storyboard"
   - ".sublime-snippet"

samples/XML/Robots.slnx

@@ -0,0 +1,10 @@
+<Solution>
+  <Project Path="build\Robots.Build\Robots.Build.csproj" />
+  <Project Path="src\Robots.Grasshopper\Robots.Grasshopper.csproj">
+    <BuildDependency Project="src\Robots\Robots.csproj" />
+  </Project>
+  <Project Path="src\Robots\Robots.csproj" />
+  <Project Path="tests\Robots.Tests\Robots.Tests.csproj">
+    <BuildDependency Project="src\Robots\Robots.csproj" />
+  </Project>
+</Solution>

test/test_strategies.rb

@@ -190,7 +190,8 @@ def test_xml
       "#{samples_path}/XML/some-ideas.mm",
       "#{samples_path}/XML/GMOculus.project.gmx",
       "#{samples_path}/XML/obj_control.object.gmx",
-      "#{samples_path}/XML/MainView.axaml"
+      "#{samples_path}/XML/MainView.axaml",
+      "#{samples_path}/XML/Robots.slnx",
     ]
     assert_all_xml all_xml_fixtures("*") - no_root_tag