feat(component): add ComposedModal

Closes #13

src/components/ComposedModal/ModalBody.svelte

@@ -0,0 +1,22 @@
+<script>
+  let className = undefined;
+  export { className as class };
+  export let hasForm = false;
+  export let hasScrollingContent = false;
+  export let style = undefined;
+
+  import { cx } from '../../lib';
+
+  const _class = cx('--modal-content', hasForm && '--modal-content--with-form', className);
+</script>
+
+<div
+  tabindex={hasScrollingContent ? '0' : undefined}
+  role={hasScrollingContent ? 'region' : undefined}
+  class={_class}
+  {style}>
+  <slot />
+</div>
+{#if hasScrollingContent}
+  <div class={cx('--modal-content--overflow-indicator')} />
+{/if}