Package for Flatpak

[Grafcube]

Closes #737

This patch adds flatpak support. I built and installed it on my branch with these commands:

pnpm build
pnpm electron-builder --linux flatpak
flatpak install --bundle 'pack/YouTube Music-3.3.6-x86_64.flatpak'

A few things to note, I need to write a separate yml to publish to flathub which repeats a lot of metadata (electron builder doesn't currently have any way to publish to flathub automatically). According to the flatpak app submission guidelines:

Flathub is primarily intended as a service that is used by app developers to distribute their apps. Our goal is to give developers control of their apps and to allow them a closer relationship with their users without middlemen getting in the way. However, as part of setting up Flathub, some applications are being distributed on Flathub without the involvement of their developers. We would prefer that these applications are controlled by their authors.

I am perfectly fine with maintaining the flatpak myself.

The appstream metadata should probably contain content warnings since YT music also has explicit content. I've left it blank for now. It also needs to be packaged manually since electron builder doesn't yet support adding it (see electron-userland/electron-builder#1993). I'll manually include it in the manifest until that issue is resolved. The appstream file is useful for all linux packages, not just flatpak, which is why I've included it in the PR.

If you don't want to wait, you can checkout my branch and install it yourself with the above commands. Cheers!