Observation of electroweak W+W− pair production in association with two jets in proton-proton collisions at <math altimg='"si1.svg"'> <msqrt> <mrow> <mi>s</mi> </mrow> </msqrt> <mo linebreak='"goodbreak"' linebreakstyle='"after"'>=</mo> <mn>13</mn> <mspace width='"0.2em"'></mspace> </math>