function createAttachmentKey() {
	return Symbol(ATTACHMENT_KEY);
}
Creates an object key that will be recognised as an attachment when the object is spread onto an element,
as a programmatic alternative to using `{@attach ...}`. This can be useful for library authors, though
is generally not needed when building an app.

```svelte
<script>
	import { createAttachmentKey } from 'svelte/attachments';

	const props = {
		class: 'cool',
		onclick: () => alert('clicked'),
		[createAttachmentKey()]: (node) => {
			node.textContent = 'attached!';
		}
	};
</script>

<button {...props}>click me</button>
```
@since 5.29