feat(viewer): allow async handler components

[pulsejet]

Allows use of async handlers

OCA.Viewer.registerHandler({
	id: 'text',
	component: () => import('./components/ViewerComponent.vue'),
})

For nextcloud/text#4698

[juliusknorr]

Works like a charm, thanks for that 👍

[pulsejet]

/backport to stable28

[github-actions]

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