feat(calendar): Add Nextcloud and iCal calendar services with event parsing

[TheItsNameless]

Description

As a regular user of Homer, I would like to see my recent and upcoming events directly on my dashboard.

With this PR, two new Services are added, the ICalendar and the NextcloudCalendar:

• The ICalendar receives a URL of an iCal-Feed and parses it to show upcoming events.
• The NextcloudCalendar requests a CalDav calendar from a Nextcloud instance calendar and parses it to show upcoming events.

The events look like this:

To save on the projects' complexity, I have not used a package for parsing the iCal, but rather relied on simple regex's. This is pretty crude but it works!

I have also added a .prettierrc.json as I had trouble with VS Code trying to format all files in a different format than what is used by default in Homer. Now others don't have to struggle, as Prettier will work better. If this is a problem, feel free to tell me, I will remove the .prettierrc.json.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I've read & comply with the contributing guidelines
• I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• I have made corresponding changes to the documentation (README.md).
• I've checked my modifications for any breaking changes, especially in the config.yml file