Make error position tracking explicit with Offset field on ParserError

Thread byte offset information through all error creation sites,
eliminating the need for SubsliceOffset to recover position from
pointer comparison.

Changes:
- Add Offset field to ParserError struct
- Add offset parameter to NewParserError
- Add Parser.offsetOf helper for suffix-length arithmetic
- Thread base offset through scanner functions (scanComment,
  scanBasicString, scanMultilineBasicString, scanLiteralString,
  scanMultilineLiteralString, scanWindowsNewline)
- Thread base offset through standalone functions (expect, hexToRune)
- Thread base offset through all decode functions (parseInteger,
  parseFloat, parseLocalDate, parseLocalTime, parseLocalDateTime,
  parseDateTime, checkAndRemoveUnderscores*)
- Update all unmarshaler call sites to pass value.Raw.Offset
- Update localtime.go UnmarshalText methods with base=0
- Update strict.go to populate Offset from key ranges
- Change wrapDecodeError to read de.Offset directly
- Change Utf8TomlValidAlreadyEscaped to return int index (-1 if valid)
  instead of a byte subslice
- Unexport SubsliceOffset (now only used internally by Range())

This makes error positions self-describing: each ParserError carries its
own byte offset, so callers no longer need the original document slice
and address arithmetic to determine where an error occurred.

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

unstable/ast.go

@@ -90,10 +90,7 @@ type Range struct {
 	Length uint32
 }
 
-// SubsliceOffset returns the byte offset of subslice within data.
-// Subslice must be a subslice of data, meaning it must point into the
-// same backing array. Panics if subslice is not within data.
-func SubsliceOffset(data []byte, subslice []byte) int {
+func subsliceOffset(data []byte, subslice []byte) int {
 	if len(subslice) == 0 {
 		return len(data)
 	}