fix(Parcel): Do not try to unmount when parcel is already unmounted

[AckermannJan]

Problem
We are currently experiencing an issue where whenever SingleSPA is unmounting the Micro-Frontend which contains a parcel we get the following error basically telling us that we are trying to unmount a parcel which is already unmounted.

Investigation
After some investigation we found out that the parcel component is not checking if the parcel is actually mounted b
efore unmounting it. Using log points in chrome we confirmed our suspicion that it is coming from single-spa-vue.

This log is emitted right before trying to call the unmount function.

Solution
Add a check to make sure that the parcel is mounted before unmounting it. While checking the single-spa-react package we saw that they have implemented the same solution, most likely for the same issue, as you can see here.

[AckermannJan]

Hey @joeldenning, sorry to ping you directly, but would you be willing to do the review for this MR?