Blacklist non-.vvvvvv files in level metadata

While the game does support playing levels with filenames that don't
have the .vvvvvv extension, it doesn't do it well.

Namely, those files can't be loaded or saved into the editor (because a
.vvvvvv always gets tacked on to your input when saving or loading). In
2.3, this gets worse because you can't load a level without a .vvvvvv
extension from command-line playtesting (because a .vvvvvv automatically
gets added) and you can't load per-level custom assets.

The only place where extensionless level files are supported is when
loading level metadata. But this makes it so they no longer work. This
is technically an API break, but it's easily fixed (just add the
.vvvvvv), plus there's nothing to be gained from not having an
extension, plus basically no one ever actually did this in the first
place (as far as I know, I am the only person to have ever done this,
and no one else ever has).

desktop_version/src/editor.cpp

@@ -212,7 +212,9 @@ static void levelMetaDataCallback(const char* filename)
     LevelMetaData temp;
     std::string filename_ = filename;
 
-    if (!FILESYSTEM_isFile(filename) || FILESYSTEM_isMounted(filename))
+    if (!endsWith(filename, ".vvvvvv")
+    || !FILESYSTEM_isFile(filename)
+    || FILESYSTEM_isMounted(filename))
     {
         return;
     }