Distinguish unmapped indentation from other unmapped content

[stof]

Is your feature request related to a problem? Please describe.
When generating sourcemaps for its expanded output, dart-sass is leaving all indentation unmapped. This results in a large amount of unmapped content, without being able to figure easily whether we have relevant unmapped content.

Describe the solution you'd like
It would be nice to separate unmapped indentation (defined as whitespace content at the beginning of the line) from other unmapped content and report the percentages separately

[nikolay-borzov]

I think this issue should be fixed on dart-sass side.
source-map-explorer can't say what unmapped content refers to.

unmappedBytes = totalBytes - mappedBytes - sourceMapCommentBytes - eolBytes;