static insertAt(str, char, index) {
        str = String(str);

        if (index > str.length) {
            throw new Error(`The given index is out of the string range.`);
        }

        if (char.length !== 1) {
            throw new Error('The given string `char` should be only one character long.');
        }

        if (str === '' && index === 0) {
            return char;
        }

        return `${str.slice(0, index)}${char}${str.slice(index)}`;
    }
Insert the single character `char` in the string `str` at the given position `index`

@param {string} str
@param {string} char
@param {int} index
@returns {string}