Only update `useSupabaseUser` state if user has actually changed

[aaaaahaaaaa]

Currently, the useSupabaseUser composable is updating the state on every single page start. This means that watching the ref returned by the composable, as shown in through out the doc, will always fire needlessly:

const user = useSupabaseUser()
watch(user, async (newUser) => {
 // always fires on page change
}

Which makes implementing any kind of other states (e.g. stores) based on that composable unpractical as it would be very completely unoptimal (e.g. firing database calls on every page load).

Therefore, the composable should only update the state if the user actually changes.