feat(types): expose ComponentCustomOptions for declaring custom options

test-dts/componentTypeExtensions.test-d.ts

@@ -1,7 +1,11 @@
-import { expectError } from 'tsd'
+import { expectError, expectType } from 'tsd'
 import { defineComponent } from './index'
 
 declare module '@vue/runtime-core' {
+  interface ComponentCustomOptions {
+    test?(n: number): void
+  }
+
   interface ComponentCustomProperties {
     state: 'stopped' | 'running'
   }
@@ -9,6 +13,11 @@ declare module '@vue/runtime-core' {
 
 export const Custom = defineComponent({
   data: () => ({ counter: 0 }),
+
+  test(n) {
+    expectType<number>(n)
+  },
+
   methods: {
     aMethod() {
       expectError(this.notExisting)