A source map kept in structured form until it is actually emitted, so that rewriting or
embedding a map shares its ropes instead of copying its bytes. sourcesContent in
particular (the full original source text) would otherwise be copied for every chunking
context that rewrites the sources URLs and for every chunk source map that embeds the
module’s map.
Implements the source map specification as a [decoded][DecodedMap] sourcemap. A “decoded” map
represents a source map as if it was came out of a JSON decode.