Add cover option to image container component

[bartaz]

Done

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

Fixes WD-12281

QA

• 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

Check if PR is ready for release

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

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• 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
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.

Screenshots

[webteam-app]

Demo

Jenkins

demos.haus

[carkod]

Looks pretty good to me.