feat: Add D language test fixture and update test mapping

tests/fixtures/languages/d/test.d

@@ -0,0 +1,26 @@
+import std.stdio;
+
+/**
+ * Main function for the D language test file.
+ */
+void main() {
+    writeln("Hello, D language!");
+    
+    auto greeter = new Greeter("World");
+    writeln(greeter.greet());
+}
+
+/**
+ * A simple greeter class in D
+ */
+class Greeter {
+    private string name;
+    
+    this(string name) {
+        this.name = name;
+    }
+    
+    string greet() {
+        return "Hello, " ~ name ~ "!";
+    }
+}