Replace `portal-vue` plugin with built-in teleport

[JammingBen]

Vue3 has introduced a built-in teleport feature that makes the portal-vue dependency obsolete. We should utilize that and drop the dependency.

Some of the existing usages of portal-target can/should be replaced with proper extension points.

[kulmann]

Some of the existing usages of portal-target can/should be replaced with proper extension points.

Yes, we have a custom component extension type, which doesn't even need the teleport feature, so wherever that makes sense we can define an extension point instead.