code: function checkVariable(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    // Skip `$`.
    if (tokens[i].type === TokenType.DollarSign) i++;
    else return 0;
  
    if (l = checkIdent(i)) i += l;
    else return 0;
  
    return i - start;
  }
docs: Check if token is part of a variable
  @param {number} i Token's index number
  @return {number} Length of the variable