add Serbian translation

[eevan78]

This commit adds the Serbian translation of the weechat.org.

[flashcode]

@eevan78: thanks for the new translation!

There are some errors in the po file, could you please fix before I merge this? (see the CI results)

[eevan78]

No problem @flashcode
I have fixed the issues now. I admit that I forgot to run msgcheck on the finished po file...

There is just one thing. Serbian dates have . after the year, and we use full month name, lowercase if it is written out with letters. But because of the constrained space, I stated M (short month name).
This is the reason for the error, as Y has to be followed by the dot, and that's the last character in the translated string :).

So, in order to pass the check, I have removed the dot at the end, although that's not correct.

[flashcode]

Thanks, merged!