Allow changing viewport in browser mode

[sheremet-va]

Clear and concise description of the problem

Currently it is not possible to set a viewport in browser mode, it is set to the default iframe height and width.

Suggested solution

Add an option in the config and a public API to change the viewport:

export default defineConfig({
  test: {
    browser: {
      viewport: { height: 100, width: 100 },
    },
  },
})

import { page } from '@vitest/browser/context'

test('works in a viewport', async () => {
  await page.viewport(100, 100)
  await page.viewport('iphone-6')
})

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[sheremet-va]

Fixed by #5811