type ReadSeekPeeker interface {
	Read(p []byte) (n int, err error)
	Seek(offset int64, whence int) (int64, error)
	Peek(n int) ([]byte, error)
}

s := "hello beautiful wonderful world!"
r := readseekpeeker.NewBufferedReadSeekPeeker(strings.NewReader(s), 5)

b := make([]byte, 5)
r.Read(b)

peek, _ := r.Peek(11)      // !

r.Seek(21, io.SeekCurrent) // !!

rest, _ := io.ReadAll(r)

fmt.Println(string(b), string(peek), string(rest))
// Output: hello  beautiful  world!

As of 2024-02-22, standard go packages allow either:
A) Seek() by io.ReadSeeker
B) Peek() by bufio.Reader
..but not both! This package adds that.