Consistent 4:3 ratio flag images

[jackocnr]

Currently, some flags are 15px tall e.g. Vatican City, and some are only 8px tall e.g. Qatar. And while most flags are 20px wide e.g. United Kingdom, some are only 13px wide e.g. Nepal. So if we select those 4 countries as our onlyCountries option, we end up with a situation like this:

Whereas if we switch to a different flag library, which uses consistent 4:3 ratio (as suggested by @bronisMateusz), we end up with this instead:

Which, while not being technically accurate in terms of respecting countries' flag shapes, provides a much more consistent UI experience.

This change will require updating/replacing the existing image build system, which will become much simpler!

I don't have time to implement this myself right now, but would welcome a pull request.

Ideally, the flag sprite will be output in AVIF format, though that shouldn't be a blocker to getting this out, as it can easily be added as a subsequent build step.