Text this: Precise test of lepton flavour universality in $$\varvec{W}$$ W -boson decays into muons and electrons in $$\varvec{pp}$$ pp collisions at $$\varvec{\sqrt{s}}=13\,\text {T}\text {e}\hspace{-1.00006pt}\text {V} $$ s = 13 Te V with the ATLAS detector