make JSON error byte position start at top of file
make JSON error byte position start at top of file

The `hi` and `lo` offsets in a span are relative to a `CodeMap`, but this
doesn't seem to be terribly useful for tool consumers who don't have the
codemap, but might want the byte offset within an actual file?

Resolves #35164.

aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust