V3 Proposal: Options are always objects – value is always stored as array

[sagalbot]

There's a lot of checks within the component to see if multiple is true, or if we're working with an option that is an object, not a string. Here's the select method:

/**
 * Select a given option.
 * @param  {Object|String} option
 * @return {void}
 */
select(option) {
  if (!this.isOptionSelected(option)) {
    if (this.taggable && !this.optionExists(option)) {
      option = this.createOption(option)
    }
    if(this.index) {
      if (!option.hasOwnProperty(this.index)) {
        return console.warn(
            `[vue-select warn]: Index key "option.${this.index}" does not` +
            ` exist in options object ${JSON.stringify(option)}.`
        )
      }
      option = option[this.index]
    }
    if (this.multiple && !this.mutableValue) {
      this.mutableValue = [option]
    } else if (this.multiple) {
      this.mutableValue.push(option)
    } else {
      this.mutableValue = option
    }
    this.onInput(this.mutableValue);
  }

  this.onAfterSelect(option)
},

If all options are objects and mutableValue is always an array, we can cut out a whole bunch of conditionals. Additionally, if each option object always has a label and an index, we don't have to check for that either. The code becomes much more concise now:

/**
 * Select a given option.
 * @param  {Object} option
 * @return {void}
 */
select (option) {
  if (!this.isOptionSelected(option)) {
    if (this.taggable && !this.optionExists(option)) {
      option = this.createOption(option);
    }
    
    if (!option.hasOwnProperty(this.index)) {
      return console.warn(
        `[vue-select warn]: Index key "option.${this.index}" does not` +
        ` exist in options object ${JSON.stringify(option)}.`,
      );
    }

    this.mutableValue.push(option);
    this.onInput(this.mutableValue);
  }

  this.onAfterSelect(option);
},