types(Form): add FormInstance type (#9139)

* types(Form): add FormInstance type

* types: useExpose type

src/form/README.md

@@ -543,6 +543,19 @@ Use [ref](https://v3.vuejs.org/guide/component-template-refs.html) to get Form i
 | resetValidation | Reset validation | _name?: string \| string[]_ | - |
 | scrollToField | Scroll to field | _name: string, alignToTop: boolean_ | - |
 
+### Types
+
+Get the type definition of the Form instance through `FormInstance`.
+
+```ts
+import { ref } from 'vue';
+import type { FormInstance } from 'vant';
+
+const formRef = ref<FormInstance>();
+
+formRef.value?.submit();
+```
+
 ### Slots
 
 | Name    | Description  |