ENH: adjust fileName methods for similarity to std::filesystem::path

- stem(), replace_name(), replace_ext(), remove_ext() etc

- string::contains() method - similar to C++23 method

  Eg,
      if (keyword.contains('/')) ...
  vs
      if (keyword.find('/') != std::string::npos) ...

applications/utilities/surface/surfaceTransformPoints/surfaceTransformPoints.C

@@ -64,13 +64,12 @@ using namespace Foam::coordinateRotations;
 
 static word getExtension(const fileName& name)
 {
-    word ext(name.ext());
-    if (ext == "gz")
-    {
-        ext = name.lessExt().ext();
-    }
-
-    return ext;
+    return
+    (
+        name.has_ext("gz")
+      ? name.stem().ext()
+      : name.ext()
+    );
 }