Define behavior of pressing enter in inline editing host

There are tentative tests for this:
https://github.com/web-platform-tests/wpt/blob/master/editing/other/insertparagraph-in-inline-editing-host.tentative.html
https://wpt.fyi/results/editing/other?label=experimental&label=master&product=chrome&product=firefox&product=safari&aligned&view=interop&q=label%3Ainterop-2023-webcompat

The tests are part of the Interop 2023 Web Compat focus area, but there's no spec for it.

If the behavior can be written down in some specification (whatever its standards track status) managed by the Editing WG, it will be much easier to implement something in Chromium that we can be confident matches the consensus of the WG.

(To reverse engineer the exact behavior from the tests isn't generally possible unless tests cover all possibilities.)