Vue3

[devmount]

This change adds support for Vue.js v3 and upgrades all other dependencies to their latest version as well. Some things to note:

• I didn't changed any styling or appearance. Should look exactly the same as before.
• I kept the options API of Vue for convenience. But if this PR is merged, shouldn't be a problem to convert to Composition API if needed
• I tested the frontend on my site and it works. For some reasons I wasn't able to test together with the FastAPI backend. So when reviewing this PR, please test the frontend AND the backend thoroughly
• The package babel-eslint is now @babel/eslint-parser
• The package vuex-persistedstate isn't maintained anymore, so I removed it without replacement. If you want its functionality, please implement an alternative.
• The package vue-template-compiler isn't needed anymore with Vue3.

So I hope this is useful for all who find this project as helpful as me.

Implements #3