More explicit documentation about 0 vs 1 based lines

[benmccann]

It's unclear to me when it says originalPositionFor uses 1-based lines if that's referring only to the input or to the output as well.

It might be nice to add a code comment to each occurrence of line in types.ts, which indicates whether it's a 1-based line or 0-based line.

[dummdidumm]

Judging from the "drop in replacement for source-map" statement it should be 1-based as the corresponding method is 1-based

[jridgewell]

Both the inputs and outputs are 1-based, except for traceSegment which uses 0-based for input/output. If you want open a PR, I'm happy to accept it.