Upgrade

Features:

1. Added search for options.
2. Changed structure for select:

<div class="nativejs-select">
  <button class="nativejs-select__placeholder">
    <span class="nativejs-select__placeholder_fixed">Favorite framework:</span>
    React
  </button>
  <ul class="nativejs-select__options">
    <li class="nativejs-select__option">
      <button class="nativejs-select__option_btn">React</button>
    </li>
    <li class="nativejs-select__option">
      <button class="nativejs-select__option_btn">Vue</button>
    </li>
    <li class="nativejs-select__option">
      <button class="nativejs-select__option_btn">Svelte</button>
    </li>
  </ul>
</div>

to

  <div class="nativejs-select nativejs-select_active">
    <button type="button" class="nativejs-select__placeholder">
      <span class="nativejs-select__placeholder-fixed">Favorite framework:</span>
      <div class="nativejs-select__placeholder-value">
        React
      </div>
    </button>
    <div class="nativejs-select__dropdown">
      <div class="nativejs-select__options">
        <button type="button" class="nativejs-select__option">React</button>
        <button type="button" class="nativejs-select__option">Vue</button>
        <button type="button" class="nativejs-select__option">Svelte</button>
      </div>
    </div>
  </div>

3. Added trigger change event for native html select.
4. Added full test code.