Create a file association with `*.ase` and `*.aseprite` file types

This flatpak needs to install a file association for `image/x-aseprite` to recognize `*.ase` and `*.aseprite` files so we can double-click to open them with the LibreSprite app.

The .desktop already lists an association with this MIME type.
But the export/share/mime folder for this flatpak is empty on my system (so maybe the association only works if one also has Aseprite installed, and that's why you didn't catch this on tests?).

I found an XML in the upstream repo: https://github.com/LibreSprite/LibreSprite/blob/master/desktop/mime/aseprite.xml
Just note that it's identical to the XML from the aseprite repo, including `` which might need to be changed to the libresprite icon (or a custom one based on it?). Possibly needs to update this upstream too at libresprite repo, but I dunno (I didn't try "installing" the AppImage to see if it somehow changes file associations correctly)