An abundance of fossil fuels combined with advances in technology to harness wind and solar power has sent energy prices crashing around the world. WSJ explains how it all happened at once. Photo illustration: Carlos Waters/WSJ Exxon Mobil Corp. is retreating from a plan to increase spending to boost its oil and gas production by 2025 and preparing to slash the book value of its assets by up to $20 billion, as the struggling company reassesses its next decade. The Texas oil giant, which has lost more than $2.3 billion over the first three quarters of this year after the coronavirus wreaked havoc on fossil-fuel demand, released a reduced spending outlook Monday for the next five years. It now plans to spend $19 billion or less next year and $20 billion…