feat(runtime-core): support config.optionMergeStrategies

Note the behavior is different from Vue 2:
- merge strategies no longer apply to built-in options.
- the default value is now an empty object and no longer exposes merge
  strategies for built-in options.

packages/runtime-core/src/componentProxy.ts

@@ -6,7 +6,8 @@ import {
   ExtractComputedReturns,
   ComponentOptionsBase,
   ComputedOptions,
-  MethodOptions
+  MethodOptions,
+  resolveMergedOptions
 } from './apiOptions'
 import { ReactiveEffect, UnwrapRef } from '@vue/reactivity'
 import { warn } from './warning'
@@ -61,7 +62,7 @@ const publicPropertiesMap: Record<
   $parent: i => i.parent,
   $root: i => i.root,
   $emit: i => i.emit,
-  $options: i => i.type,
+  $options: i => (__FEATURE_OPTIONS__ ? resolveMergedOptions(i) : i.type),
   $forceUpdate: i => () => queueJob(i.update),
   $nextTick: () => nextTick,
   $watch: __FEATURE_OPTIONS__ ? i => instanceWatch.bind(i) : NOOP