LibLine: Replace call to vfork() with fork()

We don't actually have a non-trivial vfork implementation, so just
call fork(). As a bonus, vfork() is deprecated in XCode 13.1 when
targeting macOS Big Sur, so this removes a blocker from updating our
macOS CI version.

Userland/Libraries/LibLine/InternalFunctions.cpp

@@ -553,10 +553,10 @@ void Editor::edit_in_external_editor()
     };
 
     Vector<const char*> args { editor_command, file_path, nullptr };
-    auto pid = vfork();
+    auto pid = fork();
 
     if (pid == -1) {
-        perror("vfork");
+        perror("fork");
         return;
     }