79 Keyboard scrolling support

[ae9is]

Fixes #79

The wrong element was being used to calculate the position of the font within the popout scroll. (The font preview element was being used rather than the font picker option.)

Example structure:

<div class="fontpicker" data-testid="default-fontpicker">
  <div class="fontpicker__preview font-preview-almarai"></div>
  <input class="fontpicker__search" type="text" value="">
  <div tabindex="-1" class="fontpicker__popout" role="listbox">
    <div class="fontpicker__option" role="option">
      <div class="font-preview-42dot_sans"></div>
    </div>
    <div class="fontpicker__option" role="option">
      <div class="font-preview-abeezee"></div>
    </div>
    <div class="fontpicker__option" role="option">
      <div class="font-preview-abel"></div>
    </div>
...