Add sprite user data when exporting documents

[skypjack]

I agree that my contributions are licensed under the Individual Contributor License Agreement V4.0 ("CLA") as stated in https://github.com/igarastudio/cla/blob/main/cla.md

I have signed the CLA following the steps given in https://github.com/igarastudio/cla#signing

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As discussed here on discord, sprite user data are not exported when creating the json file.
This PR adds them to the "meta" section, as in:

 "meta": {
  "app": "https://www.aseprite.org/",
  "version": "1.x-dev",
  "image": "test.png",
  "format": "RGBA8888",
  "size": { "w": 64, "h": 64 },
  "scale": "1",
  "documents": [
   { "name": "test.aseprite", "data": "my data" }
  ],
  "frameTags": [ ... ],
  "layers": [ ... ],
  "slices": [ ... ]
 }

I've no idea if this is a correct result for the spirit of this software but I had to start somewhere, right? 🙂
Let me know if this doesn't meet your tastes and what I can do to have this data in the final json.
I find this data to be pointless otherwise since one cannot export and thus use them properly.

[dacap]

Hi @skypjack! thanks for your contribution, I have to think a little more about the output format. For a one sprite -> sprite sheet, it looks like a "data" field instead of the "documents" might be preferable.

For multiple sprites -> texture atlas, at the moment we have the same issues with tags/layers/slices info from those multiple sprites/docs. Not sure what would be the best solution in these cases.

[skypjack]

Let me know if you want me to change anything in the PR. 👍