Use SourceMapGenerator for Result#map
Better errors on previous map reading
Add PreviousMap#source
Add .withSources and .object() to PreviousMap and rename .map to .text
Better PreviousMap#withSources test
Set sourcesContent to map
Rename .object() to .consumer() and .content to .withContent() in PreviousMap
Check source map object by mappings property
Add mapContent docs