Database of extensions format

We should have a file format for the database of extensions. Could we just the `package.json` of each extension? Probably we need some extra information (purchase/download link, checksum).

Some points we need/want:

1. Modifications (new extensions/new versions/deleted extensions) in the registry must be done through PRs
2. Extensions might be downloaded directly (if a link is provided) or a link to purchase it (e.g. itch)
3. We should have a checksum to check that the extension is the same that the registered one (probably only useful if the extension can be downloaded directly)

It would be nice to be able to register a simple script from its GitHub repository: The README can has some information about the extension, there could be a .lua script, and the GitHub repo SHA can act as a checksum for the specific published version.

I think the easier path to achieve access this registry of extensions is:

1. Create a `fetch()`-like method to send HTTPS requests from Lua scripts
2. Create a minimal "Aseprite Extensions" extension to access this DB
3. Add the same "Aseprite Extensions" in this DB (so we can update it from here)
4. Register the first third-party extension
5. Call for other authors to register their own extensions here