constructor(source, varsArr = []) {
    this.source = source;
    this.varsArr = varsArr;
    this.processedSource = null;
    this.extractedVars = null;
    this.extractedProps = null;
    this.calculatedVars = null;
    this.calculatedProps = null;
  }
Constructor
@param {Object} source plain object style with variables
@param {Array} [varsArr] array of vars objects