Linux: Embed AppImage update information

Request to provide `.zsync` file for AppImage delta updates.
Updating will take less amount of data and time due to downloading only `diff`.

info:
- https://github.com/AppImageCommunity/AppImageUpdate#update-information-overview
- https://github.com/AppImage/AppImageSpec/blob/master/draft.md#update-information
- https://github.com/ivan-hc/AppImage-tips#how-to-add-delta-updates

This is a common standard for updating AppImages.
AppImage having update information would also recognized by other software e.g. [AppImageLauncher](https://github.com/TheAssassin/AppImageLauncher), [AppImageUpdate](https://github.com/AppImageCommunity/AppImageUpdate), [AppManager](https://github.com/kem-a/AppManager), [AM](https://github.com/ivan-hc/AM) etc...

Lots of projects already support updating AppImage e.g: [LibreWolf](https://librewolf.dev/librewolf/bsys6/releases), [KeePassXC](https://github.com/keepassxreboot/keepassxc/releases), [Sigil](https://github.com/Sigil-Ebook/Sigil/releases) etc.

Start by reading the linked AppImage update-information specifications and the release packaging flow for CopyQ's Linux AppImage. Determine where the distributed AppImage is assembled and how its release artifacts are produced; done means the AppImage includes valid update information and a corresponding .zsync artifact that the referenced update tools can recognize.

Written by the indexing model from the issue text.