Rename serializable to shadowrootserializable

See the conversation here:
  https://github.com/whatwg/html/pull/10139

But the TL;DR is that the `<template>` attribute corresponding
to the declarative shadow root `serializable` attribute should
be named in a parallel way to other DSD-related attributes,
meaning it should start with `shadowroot*`.

Bug: 41490936
Change-Id: I40ea496c22147bac7dcdfb74cd7bf4a3abae8be9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5375416
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1274374}