High-pressure \({\mathrm{Na}}_{3}{\left({\mathrm{N}}_{2}\right)}_{4},\) \({\mathrm{Ca}}_{3}{\left({\mathrm{N}}_{2}\right)}_{4},\) \({\mathrm{Sr}}_{3}{\left({\mathrm{N}}_{2}\right)}_{4},\) and \(\mathrm{Ba}{\left({\mathrm{N}}_{2}\right)}_{3}\) featuring nitrogen dimers with noninteger charges and anion-driven metallicity