fix(viewer): mount text vue instance explicitely

[max-nextcloud]

The component configured in the viewer handler
is instantiated from the vue module inside the viewer.

We want to use our own vue module so create an instance explicitely and mount it.

This fixes warnings in development like $attrs is readonly.

It also makes the vue dev tools work!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[max-nextcloud]

🖼️ Dev tools Screenshots

🏚️ Before	🏡 After

[juliusknorr]

I'm unsure if we have a test for #5531 these days but should double check this case

[max-nextcloud]

I'm unsure if we have a test for #5531 these days but should double check this case

We do have this:

text/cypress/e2e/attachments.spec.js

Lines 301 to 330 in 302dcc7

	it('Create a new text file as an attachment', () => {
	const check = (documentId, fileName) => {
	cy.log(
	'Check the attachment is visible and well formed',
	documentId,
	fileName,
	)
	return cy
	.get(`.text-editor [basename="${fileName}"]`)
	.find('.text-editor__wrapper')
	.should('be.visible')
	}
	cy.visit('/apps/files')
	cy.openFile('test.md')
	cy.log('Create a new text file as an attachment')
	const requestAlias = 'create-attachment-request'
	cy.intercept({ method: 'POST', url: '**/text/attachment/create' }).as(
	requestAlias,
	)
	clickOnAttachmentAction(ACTION_CREATE_NEW_TEXT_FILE).then(() => {
	return waitForRequestAndCheckAttachment(
	requestAlias,
	undefined,
	false,
	check,
	)
	})
	})

( I know because it was broken initially.) The editor for the text file in a text file renders fine.