feat(runtime-core): add special property to get class component options

[ktsn]

This PR adds a special property __vueClassComponentOptions to process class style component.

If a component passed to createVNode is a function and having __vueClassComponentOptions property, it gets the actual component options from that property. To decouple with vue-class-component as possible, Vue core only knows where the options object of class component is stored. vue-class-component should convert the class to a component options object.

Vue v3 + vue-class-component is already working with this change. The brief idea of the implementation looks like below:

// Providing Vue base constructor from vue-class-component
export class Vue<Props = unknown> implements ComponentPublicInstance<{}, {}, {}, {}, {}, Props> {
  // Define a getter to convert the class to options object
  static get __vueClassComponentOptions() {
    // converting logic here...
  }
}

// ------

// Usage
class App extends Vue<{ name: string }> {
  greeting = 'Hello, ' + this.$props.name
}

[yyx990803]

Can we use a shorter key to save a few bytes? Maybe __vccOpts? (I assume this key will be fully internal and never exposed to the user)

[ktsn]

Sure, renamed.