Improved auto-completion : support utf-8

[khjde1207]

support utf-8
Even if there are Hangul or Chinese characters, the automatic completion function works.
example :

package main

import (
"fmt"
)

func main() {
fmt.Println("aaaa")
//한글테스트 아아아아아아아아아
//真的
fmt.
}

[khjde1207]

Reference link : https://stackoverflow.com/questions/5515869/string-length-in-bytes-in-javascript