Add cover option to image container component

Done

Adds image cover option to image container component, using is-cover modifier class.

Open demo
Check if the image covers the container
Change the container class to some other aspect ratio, such as p-image-container--3-2 or p-image-container--square, verify that image adapts correctly (and still covers the container)
Review updated documentation:
- https://vanilla-framework-5199.demos.haus/docs/patterns/images#cover-image

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

[x] PR should have one of the following labels to automatically categorise it in release notes:
- Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
[x] Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
- if CSS class names are not changed it can be bugfix relesase (x.x.X)
- if CSS class names are changed/added/removed it should be minor version (x.X.0)
- see the wiki for more details
[x] Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.