add sentinel errors

Add sentinel errors to allow easier checking of the type of error that
occurred during an operation.

errors.go

@@ -1,6 +1,16 @@
 package jsonpointer
 
-import "strconv"
+import (
+	"errors"
+	"strconv"
+)
+
+var (
+	ErrArrayIndexOutOfBounds = errors.New("jsonpointer: array index out of bounds")
+	ErrInvalidArrayIndex     = errors.New("jsonpointer: invalid array index")
+	ErrInvalidPointer        = errors.New("jsonpointer: invalid pointer")
+	ErrValueNotFound         = errors.New("jsonpointer: value not found")
+)
 
 type arrayIndexOutOfBoundsError struct {
 	index int
@@ -10,6 +20,10 @@ func (err *arrayIndexOutOfBoundsError) Error() string {
 	return "jsonpointer: array index out of bounds " + strconv.Itoa(err.index)
 }
 
+func (err *arrayIndexOutOfBoundsError) Is(target error) bool {
+	return target == ErrArrayIndexOutOfBounds
+}
+
 type invalidArrayIndexError struct {
 	tok string
 }
@@ -18,6 +32,10 @@ func (err *invalidArrayIndexError) Error() string {
 	return "jsonpointer: invalid array index " + strconv.QuoteToASCII(err.tok)
 }
 
+func (err *invalidArrayIndexError) Is(target error) bool {
+	return target == ErrInvalidArrayIndex
+}
+
 type invalidPointerError struct {
 	ptr string
 }
@@ -26,6 +44,10 @@ func (err *invalidPointerError) Error() string {
 	return "jsonpointer: invalid pointer " + strconv.QuoteToASCII(err.ptr)
 }
 
+func (err *invalidPointerError) Is(target error) bool {
+	return target == ErrInvalidPointer
+}
+
 type invalidTokenError struct {
 	tok string
 }
@@ -34,10 +56,18 @@ func (err *invalidTokenError) Error() string {
 	return "jsonpointer: invalid token in pointer " + strconv.QuoteToASCII(err.tok)
 }
 
+func (err *invalidTokenError) Is(target error) bool {
+	return target == ErrInvalidPointer
+}
+
 type valueNotFoundError struct {
 	tok string
 }
 
 func (err *valueNotFoundError) Error() string {
 	return "jsonpointer: value not found " + strconv.QuoteToASCII(err.tok)
 }
+
+func (err *valueNotFoundError) Is(target error) bool {
+	return target == ErrValueNotFound
+}

get.go

@@ -124,7 +124,7 @@ func (p Pointer) Get(value any) (any, error) {
 	}
 
 	refResult := reflect.ValueOf(result)
-	for i, tok = range p.tokens[i:] {
+	for _, tok = range p.tokens[i:] {
 		if err := getReflect(tok, &refResult); err != nil {
 			return nil, err
 		}