Absorption spectra and isotope shifts of the (2, 0), (3, 1), and (8, 5) bands of the ${{\rm{A}}}^{2}{{\rm{\Pi }}}_{{\rm{u}}}\unicode{x2013}{{\rm{X}}}^{2}{{\boldsymbol{\Sigma }}}_{g}^{+}$ system of ${}^{15}{\rm{N}}_{2}^{+}$ in near infrared