Remove Parser.Range and subsliceOffset

Range() existed to recover byte offsets from Highlight subslices.
Now that ParserError carries an explicit Offset field, Range() is
unnecessary. Remove it along with the private subsliceOffset helper
in ast.go.

Tests now use perr.Offset directly and construct Range literals
for Shape() calls.

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

unstable/parser.go

@@ -70,18 +70,6 @@ func (p *Parser) offsetOf(b []byte) int {
 	return len(p.data) - len(b)
 }
 
-// Range returns a range description that corresponds to a given slice of the
-// input. If the argument is not a subslice of the parser input, this function
-// panics.
-//
-// Prefer using ParserError.Offset directly for error position information.
-func (p *Parser) Range(b []byte) Range {
-	return Range{
-		Offset: uint32(subsliceOffset(p.data, b)), //nolint:gosec // TOML documents are small
-		Length: uint32(len(b)),                    //nolint:gosec // TOML documents are small
-	}
-}
-
 // rangeOfToken computes the Range of a token given the remaining bytes after the token.
 // This is used when the token was extracted from the beginning of some position,
 // and 'rest' is what remains after the token.