feat(reactive): allow usage of reactive before Vue.use

[pikax]

Allowing define reactive and ref before the Vue.use, the only caveat is when you have different vue versions installed.

I think this is a pretty edge case and most of the time the user will use the version from required('vue')

Note: would be nice to validate if reactive was used and if it was before the Vue.use and the Vue.use resolves to a different Vue constructor we could warn it

// something like this
export function setVueConstructor(Vue: VueConstructor) {
  if (__DEV__ && vueConstructor) {
    warn('Another instance of vue installed')
  }

  if(__DEV__ && !vueConstructor && hasReactiveBeenUsed && vueConstructor !== vueDependency) {
    warn('reactive was used globally with a different Vue instance, this might cause reactivity to fail')
  }
  vueConstructor = Vue
  Object.defineProperty(Vue, PluginInstalledFlag, {
    configurable: true,
    writable: true,
    value: true,
  })
}

[antfu]

This is definitely good to have. Not sure if there are any caveats for bundles though, guess we can just revert it if something really goes wrong. LGTM 👍