code: function checkPlaceholder(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    if (tokens[start].placeholder_l) return tokens[start].placeholder_l;
  
    // Skip `%`.
    if (tokens[i].type === TokenType.PercentSign) i++;
    else return 0;
  
    if (l = checkIdentOrInterpolation(i)) {
      i += l;
      tokens[start].placeholder_l = i - start;
    } else return 0;
  
    return i - start;
  }
docs: Check if token is part of a placeholder selector (e.g. `%abc`).
  @param {number} i Token's index number
  @return {number} Length of the selector