feat(form): add ref prop (#900)

Closes #877

src/Form/Form.svelte

@@ -1,6 +1,12 @@
+<script>
+  /** Obtain a reference to the form element */
+  export let ref = null;
+</script>
+
 <!-- svelte-ignore a11y-mouse-events-have-key-events -->
 <form
   class:bx--form="{true}"
+  bind:this="{ref}"
   {...$$restProps}
   on:click
   on:keydown