_readSelection(filePath, line, startColumn, endColumn) {
    const code = fs.readFileSync(filePath).toString();
    const lines = code.split('\n');
    const selectionLine = lines[line - 1];
    const tmp = [];
    for (let i = startColumn; i < endColumn; i++) {
      tmp.push(selectionLine.charAt(i));
    }
    return tmp.join('');
  }
read selection text in file.
@param {string} filePath - target file full path.
@param {number} line - line number (one origin).
@param {number} startColumn - start column number (one origin).
@param {number} endColumn - end column number (one origin).
@returns {string} selection text
@private