A proposal to expand a major local pipeline could bring cheap, plentiful supplies of natural gas to New England from Northeast shale formations, but it also thrusts the region into a raging debate that pits economics against environment, industry against community, and sometimes neighbor against neighbor. The focus is often on a controversial technique, known as fracking, that pumps chemical-laced, pressurized water deep into the earth. The technology has helped bring jobs, businesses, and money, but also been blamed for polluting air and water.