test: add TS types

tests/Modal.test.svelte

@@ -0,0 +1,20 @@
+<script lang="ts">
+  import { Button, Modal } from "../types";
+
+  let open = false;
+</script>
+
+<Button on:click="{() => (open = true)}">Create database</Button>
+
+<Modal
+  bind:open
+  modalHeading="Create database"
+  primaryButtonText="Confirm"
+  secondaryButtonText="Cancel"
+  on:click:button--secondary="{() => (open = false)}"
+  on:open
+  on:close
+  on:submit
+>
+  <p>Create a new Cloudant database in the US South region.</p>
+</Modal>