Add devcontainer for codespace development

[eeejay]

This is useful for easy development on a live system without compromising the developer's /usr.

[eeejay]

Just realized that bwrap won't work because we don't have kernel.unprivileged_userns_clone .
Investigating if we will need to use setuid mode here..

[eeejay]

OK, "runArgs": ["--privileged"] did the trick.

[TingPing]

I'm not against it but I think minimal effort will be made to maintain it.

[GeorgesStavracas]

I don't know what a devcontainer nor codespace are, so I must say, it would be appreciated if you could write a more detailed commit message explaining where this is expected to be used, and how, by whom, under which circumstances.

[eeejay]

I added a better commit message. If you click on the "Code" dropdown button in the main flatpak repo page there is a "Codespaces" tab. If you proceed to create a codespace a new tab will open with VSCode loaded into it with a full dev environment. The .devcontainer file I provided installs all the prerequisites and configures the container for Flatpak development.

One of the nice things about this is that it allows you to set up a VM very quickly so you can install Flatpak to /usr and mess around to your heart's content.

[GeorgesStavracas]

Thanks for improving the commit message as requested!