The subsliceOffset method incorrectly computed offset as
len(p.data) - len(b), which only works when b is a suffix (tail) of
p.data. However, error highlights (ParserError.Highlight) are arbitrary
subslices from the middle of the input (e.g., b[0:1] from parseSimpleKey),
so their len has no relationship to their position.
This was a regression introduced in commit 3aaf147 (Remove unsafe package
usage) which replaced danger.SubsliceOffset (pointer arithmetic) with the
incorrect len-based approach.
Fix by using reflect.ValueOf().Pointer() to compute the actual byte
offset between slice data pointers, matching the approach already used
in errors.go:subsliceOffset.
Fixes#1047
Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>
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