Add custom flag: aztec

[iqsuyoo]

Hi! I’d like to propose adding a custom flag called "aztec".

I created a few design concepts that match the DDNet flag style (simple, clean, no text). The design is minimalistic and uses turquoise/black colors inspired by my in-game skin.

Here are some examples of the flag design:

I think it would fit well with the existing flags in Tater Client.

Thanks for your time!