constructor(str, stack, parts) {
    this.str = str;
    this.stack = stack || [];
    this.parts = parts || [];
    this.pos = 0;
    this.level = 2;
    this.nested = true;
    this.ignore = false;
  }
Initialize a new `SelectorParser`
with the given `str` and selectors `stack`.

@param {String} str
@param {Array} stack
@param {Array} parts
@api private