This brings up to date with the specification for declarative shadow
roots: https://github.com/whatwg/html/pull/12267.
The `shadowrootslotassignment` attribute on `<template>` elements
specifies the slot assignment mode used by the declarative shadow root
created by the template.
Testing: New tests start to pass
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>