Presets should ignore the toplevel "$schema" entry

To utilize the VSCode’s JSON completion, The CMakePresets.json must contain the “$schema” entry at the top-level. But cmake.exe refuses this ( :disappointed_relieved: say “Invalid root object”).

In my opinion, the VSCode extension should recognize CMakePresets.json and CMakeUserPresets.json, and apply the appropriate schema automatically. Nevertheless, perhaps it does make sense to allow a $schema key anyway.