Add dependabot to keep our dependencies up to date

[halbekanne]

We could add Dependabot in the future. When there are new versions of dependencies, dependabot will bump the version and create a pull request. Normal GitHub actions would apply, meaning we could have tests that run automatically on these PRs to check if the frontend still runs as expected.

With this workflow, handling frontend dependencies should become easier and more secure.
Check out: https://dependabot.com