fix(default-flatpaks): Add error-check & validation of flatpak IDs in build-time

[fiftydinar]

This would be beneficial to avoid wrongly typed flatpak IDs in recipe & users encountering flatpaks not installing/removing due to this situation.

Local modification would also benefit from this, but that can be a bit more clunky to reliably implement, due to the fact that local modification is done on booted system.

Bluefin had the same issue where the maintainer wrongly typed the ID for Pods app, causing the constant restart of their service & increased CPU usage, which some Bluefin user noticed.

Thanks to @Oakleafknight06 for reminding me of this issue.

[fiftydinar]

I added this for flathub only, so I guess I can close this issue.

#281