feat: defineMixin() allows to write typesafe mixins.

src/defineMixin.ts

@@ -0,0 +1,68 @@
+import type {
+  ComputedOptions,
+  MethodOptions,
+  ComponentOptionsWithObjectProps,
+  ComponentOptionsMixin,
+  ComponentPropsOptions,
+  EmitsOptions,
+} from 'vue'
+
+export /** #__PURE__*/ function defineMixin<
+  PropsOptions extends Readonly<ComponentPropsOptions>,
+  RawBindings,
+  D,
+  C extends ComputedOptions = {},
+  M extends MethodOptions = {},
+  Mixin extends ComponentOptionsMixin = ComponentOptionsMixin,
+  Extends extends ComponentOptionsMixin = ComponentOptionsMixin,
+  E extends EmitsOptions = {},
+  EE extends string = string
+>(
+  options: ComponentOptionsWithObjectProps<
+    PropsOptions,
+    RawBindings,
+    D,
+    C,
+    M,
+    Mixin,
+    Extends,
+    E,
+    EE
+  >
+): typeof options {
+  return options
+}
+
+// Test
+
+const composable = defineMixin({
+  props: {},
+  data() {
+    return {
+      count: 0,
+    }
+  },
+  watch: {
+    count(count) {
+      console.log('count', count, this.count)
+    },
+  },
+  created() {
+    this.count
+  },
+  computed: {
+    doubleCount(): number {
+      return this.count * 2
+    },
+  },
+  methods: {
+    logCounts() {
+      console.log(this.count, this.doubleCount)
+      return true
+    },
+  },
+})
+
+composable.count.value
+composable.doubleCount.value
+composable.logCounts()