feat: Add Flatpak to this project

[Plarpoon]

Hello again,

I managed to introduce Flatpak support to this project. You can test and debug things using the build-flatpak.sh file, as requested in VC by @airtonix. This project fully builds the flatpak and is now entirely self-contained.

The name has been changed to com.github.chollingworth.Lampray.yml for the very same reason as above. In the next phase, this will be built using a CI/CD GitHub pipeline made by @airtonix.

I have used the OBS Flatpak manifests as a muse for my flatpak build system, which was heavily recommended to me.

Please remember that every so often before an official update, we should bump up the version in 50-p7zip.yml.

I would like to take this opportunity to ask for the merge of #122 to this branch of the repository instead of main, if agreed.

P.s.
I have 3 hours of sleep in me, I did 2 exams today at the uni and it's 03:05 AM. My body is moving on coffee alone.

I am wishing you all goodnight, please do contact me on Discord if you want to coordinate for the next phases or you want me to screenshare and explain things. I will be glad, but now I need to sleep 💤

[Plarpoon]

Forgot to say, I included the src of the project as a good practice into the container itself.

This is how it looks like inside, I have done some automatized cleanup too.