feat: Add special query to load customElements

[ericfennis]

Description: Special Query customElement

closes: #214

Special query can be used to load a Vue component as a custom element without changing the file extension or global configuration:

import { defineCustomElement } from 'vue'
import Example from './Example.vue?customElement'

// register
customElements.define('my-example', defineCustomElement(Example))

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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