add integrated transitions

[Rich-Harris]

This adds four transitions — fade, fly, draw and slide — into the Svelte repo itself. It also adds the family of easing equations that are recommended for use with transitions, animations and tween.

The module matches the name of the directive:

<script>
  import { fade } from 'svelte/transition';
  let visible = false;
</script>

<label>
  <input type="checkbox" bind:checked={visible}>
  visible
</label>

{#if visible}
  <div transition:fade>
    fades in and out
  </div>
{/if}

[Conduitry]

👍