Shadow DOM

Further investigate using Shadow DOM in the Isolated Block Editor. This should be optional.

The Chrome extension has a Shadow DOM option, and it works well for isolating the CSS, but seems to cause issues with the editor when pressing enter.

Start by reproducing the editor behavior described in the issue with the Chrome extension's Shadow DOM option, especially what happens when pressing Enter. Trace how the Isolated Block Editor uses Shadow DOM and determine whether an optional mode can be supported without the editor issue; done means the expected behavior and scope are confirmed.

Written by the indexing model from the issue text.