Add support for overflow: clip to Cover and Group blocks

[jarekmorawski]

What problem does this address?

The overflow: clip property in CSS could unlock interesting new ways to build layouts using blocks. For instance, we'd let designers position images and other decorative elements anywhere inside the Cover, Group, or another container block. Such layouts are common, especially on websites with large and prominent headers.

According to Can I Use, the property seems to have good browser support.

What is your proposed solution?

Shall we explore what such support could look like?

[jarekmorawski]

A possible UI inspiration from Blockera, an alternative block-based page building tool.

[patil-vipul]

@jarekmorawski I've verified the overflow property is not available on the latest cover and group blocks in Gutenberg.
I'll be raising a PR for adding the property.