feat(components): add Form, FormGroup, FormItem, FormLabel

Supports #17

Fixes broken storybook.

TODO: Define stories using other components (e.g. NumberInput, Select...)

src/components/FormGroup/FormGroup.svelte

@@ -0,0 +1,22 @@
+<script>
+  let className = undefined;
+  export { className as class };
+  export let legendText = '';
+  export let invalid = false;
+  export let message = false;
+  export let messageText = '';
+  export let style = undefined;
+
+  import { cx } from '../../lib';
+
+  const _class = cx('--fieldset', className);
+  const _legendClass = cx('--label', className);
+</script>
+
+<fieldset data-invalid={invalid ? '' : undefined} class={_class} {style}>
+  <legend class={_legendClass}>{legendText}</legend>
+  <slot />
+  {#if message}
+    <div class={cx('--form__requirements')}>{messageText}</div>
+  {/if}
+</fieldset>