Add seperate Json for stackable grid example

[radekrudak]

Currently inventory_grid_stacked_transfer.tscn uses protoset_grid.json, which doesn't have stack_size and max_stack_szie properties, only items already in inventory do. Which means that added items will not be stack able with each other, only with preexisting items and it makes example less clear. I personally was wondering for a while why inventory_grid_stacked_transfer and inventory_grid_transfer behave differently despite being seemingly identical (besides UI), before i realized that already existing items are different.

In opinion adding json with times that have those properties specifically for this example, would make example more clear, and communicate how add-on works better.

[radekrudak]

While i work professionally as C++ programmer for 3 years, i haven't use github in while, and it's my first time getting involved with this project, but i'm willing to do a PR, first problem i see is i'm not sure where said json file should reside, i see other ones are in test/data folder which might be misleading to put this new json there unless test that uses it would be added. Putting it in examples or examples/data folder would be out of place (since rest resides in tests/data), making separate json files for all examples and putting them in examples/data would fix that, but would leading to data duplication.