ci: Validate VS filter files in buiild

msbuild does not use the project in it's entirety to build the app.  This means problems can slip through, so an easy solution is just to ensure the XML is well-formed.  More sophisticated validation could be done with something like `xmllint` but seems overkill.

.github/workflows/scripts/validate-vs-filters.ps1

@@ -0,0 +1,19 @@
+$filterFiles = Get-ChildItem $PWD -name -recurse *.vcxproj.filters
+
+$failed = $FALSE
+foreach ($file in $filterFiles) {
+  # Skip 3rdparty files
+  if ($file -NotMatch "^3rdparty") {
+    $expression  = "python -c `"import sys, xml.dom.minidom as d; d.parse(sys.argv[1])`" $($file)"
+    $expression += ';$LastExitCode'
+    $exitCode = Invoke-Expression $expression
+    if($exitCode -ne 0){
+      Write-Host -foregroundColor red "$($file) - Invalid VS filters file.  Likely missing tags"
+      $failed = $TRUE
+    }
+  }
+}
+
+if ($failed) {
+  exit 1
+}