Experiment w/ windows URIs

Author: Drodt

key.core/src/main/java/de/uka/ilkd/key/nparser/builder/IncludeFinder.java

@@ -60,7 +60,9 @@ private void addInclude(String filename) throws MalformedURLException {
         // whatsoever
         filename = filename.replace('\\', File.separatorChar); // Special handling for Linux
         var path = base.resolve(filename).normalize();
-        var uri = URI.create(path.toString());
+        String pathString = path.toString().replace(File.separatorChar, '/'); // URIs on Windows
+                                                                              // needs slash
+        var uri = URI.create(pathString);
         if (uri.getScheme() == null) {
             uri = URI.create("file://" + path);
         }