test: add TS types

tests/ComposedModal.test.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+  import {
+    ComposedModal,
+    ModalHeader,
+    ModalBody,
+    ModalFooter,
+    Checkbox,
+  } from "../types";
+
+  let checked = false;
+</script>
+
+<ComposedModal open>
+  <ModalHeader title="Confirm changes" />
+  <ModalBody hasForm>
+    <Checkbox labelText="I have reviewed the changes" bind:checked />
+  </ModalBody>
+  <ModalFooter primaryButtonText="Proceed" primaryButtonDisabled="{!checked}" />
+</ComposedModal>