Enable source map collection in Lighthouse

[rviscomi]

Per @connorjclark:

all you'd need to do is run Lighthouse (as http archive already does) but with the -G option to save the artifacts. In artifacts.json, you read .SourceMaps to get all the maps found for the page. here's the type for it: https://github.com/GoogleChrome/lighthouse/blob/master/types/artifacts.d.ts#L290 . "RawSourceMap" is just the source map itself, as an object.