Add type inference for emitted events (#800)

* feat: TS for emitted events

* Update src/guide/typescript-support.md

Co-authored-by: skirtle <65301168+skirtles-code@users.noreply.github.com>

Co-authored-by: skirtle <65301168+skirtles-code@users.noreply.github.com>

Author: NataliaTepluhina

src/guide/typescript-support.md

@@ -212,7 +212,7 @@ const Component = defineComponent({
       type: Object as PropType<Book>,
       // Make sure to use arrow functions
       default: () => ({
-        title: "Arrow Function Expression"
+        title: 'Arrow Function Expression'
       }),
       validator: (book: Book) => !!book.title
     },
@@ -221,7 +221,7 @@ const Component = defineComponent({
       // Or provide an explicit this parameter
       default(this: void) {
         return {
-          title: "Function Expression"
+          title: 'Function Expression'
         }
       },
       validator(this: void, book: Book) {
@@ -232,6 +232,30 @@ const Component = defineComponent({
 })
 ```
 
+## Annotating emits
+
+We can annotate a payload for the emitted event. Also, all non-declared emitted events will throw a type error when called:
+
+```ts
+const Component = defineComponent({
+  emits: {
+    addBook(payload: { bookName: string }) {
+      // perform runtime validation
+      return payload.bookName.length > 0
+    }
+  },
+  methods: {
+    onSubmit() {
+      this.$emit('addBook', {
+        bookName: 123 // Type error!
+      })
+
+      this.$emit('non-declared-event') // Type error!
+    }
+  }
+})
+```
+
 ## Using with Composition API
 
 On `setup()` function, you don't need to pass a typing to `props` parameter as it will infer types from `props` component option.