Add composite api

[kevinluo201]

#122

1. Clone the Overlay.vue to OverlayOptionAPI.vue and mount it when vue === 2
2. convert Overlay.vue to Composite API
3. extract the toggle button to ToggleButton.vue

[kevinluo201]

I create this PR because it works, but I still have several questions so welcome to discuss 😁

1. Vue 2.7 has built-in compositeAPI, but I don't know how to recognize Vue version on runtime in load.js. I only mount the composite-api version when vue === 3
2. There is no this in the setup script, so only the public methods are exposed. (window as any).__VUE_INSPECTOR__ = { enable, disable, openInEditor } It's ok, isn't it?
3. I don’t know how onUpdated, onEnabled and onDisabled are used...I thought we only need to emit event or make them props?
4. I think isChildOf is not used and can be deleted, right?

[kevinluo201]

I just removed isChildOf

[devtobi]

Wow great work already! Looking forward to get this merged!