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/index.ts

@@ -163,7 +163,8 @@ export {
   AppConfig,
   AppContext,
   Plugin,
-  CreateAppFunction
+  CreateAppFunction,
+  OptionMergeFunction
 } from './apiCreateApp'
 export {
   VNode,