fix: add emits options to defineComponent(fix #553)

[Mister-Hope]

https://v3.vuejs.org/guide/component-custom-events.html#defining-custom-events

https://eslint.vuejs.org/rules/require-explicit-emits.html

TIP

It is recommended to define all emitted events in order to better document how a component should work.

Though it has no effects on Vue2, but we can still add emits: string[] | Record<string, null | ((emitData: any) => boolean) > in defineComponent for better documenting and to avoid eslint-plugin-vue@V7 throwing warnings.

Also, we have to warn people in readme that this has no effects for validating the emit event and just for smoothing the miragation.

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Adding emits options will broke ThisType in defineComponent now.

while eslint will throw warnings if using vue/require-explicit-emits