fix: reuse if there is already defined custom element

[Donggggg]

Summary of Changes

• If a custom element(em-emoji-picker) is already defined, use it to create an instance.

Reason for Changes

In emoji-mart-react, an instance is created using custom element(Picker) during initial rendering.
If my application has more than 2 packages using emoji-mart, I get into trouble in the following scenario.

1. When initializing emoji-mart in the first package, the custom element(em-emoji-picker) is defined because it has never been defined.
   

   emoji-mart/packages/emoji-mart/src/components/Picker/PickerElement.tsx

   Lines 30 to 34 in 16978d0

   	if (
   	typeof customElements !== 'undefined' &&
   	!customElements.get('em-emoji-picker')
   	) {
   	customElements.define('em-emoji-picker', PickerElement)

2. When initializing emoji-mart in the second package, the custom element(em-emoji-picker) is already defined, so it does not defined.
3. When initializing an instance using a custom element in the second package, Illegal constructor error occurs because the class used in the second package is not defined.

To fix this issue, I added guard logic to reuse custom element that is already defined.

Issues

#812