Add flatpak version

[gattolfo98]

Describe the bug
Is not a bug, but is very appreciate have a flatpak version, so on linux we will have this software on all distro.

Expected behavior
Have a flatpak version lol.

Desktop (please complete the following information):
Linux

[fgnm]

Suggestions on how to make it? :) Currently release build are made with jpackage which bundle the main jar, a jvm (17 but upgradeable to 21 as soon as libGDX will move to Java 8) and all built-in plugins (other jar files)... I'm an ubuntu-only user, sorry, but if you have resources how to do that I can see if it's easy to setup a workflow for it

[gattolfo98]

In flatpak you don't need to add a bundled jvm (or if you want yes) you can just say "my program use java 17" in the dependency and flatpak gonna take care of this. https://docs.flatpak.org/en/latest/first-build.html tutorial is how make your first flatpak, and https://www.reddit.com/r/flatpak/comments/b5vrv6/java_in_a_flatpak/ and https://discourse.flathub.org/t/how-can-i-make-flatpak-for-applications-like-i2p-java-version/3345/2 is a 2 post speaking about java in flatpak. If you want i can help you to make the flatpak (i'm also at firs approach)! I think flatpak can make easy acces for hyperlap to a lot of people (also me, now i'm on fedora, and if i want to use it, or i need a VM, or container o building from source).

[fgnm]

Building from source is quite easy btw.. if java 17 is installed in someway (system or thorugh an ide), it's literally 1 line to run it ./gradlew runHyperLap2D... Anayway, I'll look into it, just found an example made for a libGDX game too (https://github.com/flathub/com.shatteredpixel.shatteredpixeldungeon/blob/94c9c577a251eec9caa6c256a5b37a8c9efd0893/com.shatteredpixel.shatteredpixeldungeon.yaml), it should't be too much difficult to make... I've to do more pressing issues first (sadly linux users are not so much :( ) but meanwhile if you want to give a try PR are always more then welcome... Also you can join Discord server for fast chat