chore: bump api-extractor from 7.12.1 to 7.15.1

[Gavin-Gong]

api-extractor 7.12.1 reports some warnings as following image

upgrade to 7.15.1 can fix it. api-extractor 7.12.1 depend on typescript 4.1.5, but vue-next and api-extractor 7.15.1 depend on typescript 4.2.*

there are some breaking changes, typescript 4.1.5 lib.es2015.reflect.d.ts vs. typescript 4.2.4 lib.es2015.reflect.d.ts

maybe another solution, remove ownKeys return-type

-  function ownKeys(target: object): (string | symbol)[] {
+ function ownKeys(target: object) {
  track(target, TrackOpTypes.ITERATE, isArray(target) ? 'length' : ITERATE_KEY)
  return Reflect.ownKeys(target)
}