sheet(sheetNameOrIndex) {
        if (_.isInteger(sheetNameOrIndex)) return this._sheets[sheetNameOrIndex];
        return _.find(this._sheets, sheet => sheet.name() === sheetNameOrIndex);
    }
Gets the sheet with the provided name or index (0-based).
@param {string|number} sheetNameOrIndex - The sheet name or index.
@returns {Sheet|undefined} The sheet or undefined if not found.