Change the behavior of clonable to be more opt-in

See the discussion here:

https://github.com/whatwg/html/issues/10107#issuecomment-1921837943

The existing *shipped* behavior (i.e. before the `clonable` concept
was introduced) was that any declarative shadow root *within a `<template>`* would be automatically cloned, but no others.

The semi-new behavior is the `clonable` bit concept, in which all
declarative shadow roots have their `clonable` bit set to true, so
they automatically get cloned by `cloneNode()`. That's regardless of
whether they are inside or outside a template.

The new consensus is that the "semi-new" clonable behavior is likely
web-incompatible, because clones will just start getting shadow roots
included. Plus it wasn't very developer-desirable. The new consensus
is therefore to add a `shadowrootclonable` attribute for declarative shadow dom that allows a shadow root to opt-in to this behavior, but
the default for all shadow roots will be `clonable=false`.

This CL implements the new consensus behind the ShadowRootClonable
flag. If the flag is false, the "shipped" behavior will be emulated
via setting `clonable` in an equivalent way.

See these three spec PRs:
  https://github.com/whatwg/dom/pull/1246
  https://github.com/whatwg/html/pull/10069
  https://github.com/whatwg/html/pull/10117

Bug: 1510466
Change-Id: Ice7c7579094eb08b882c4bb44f93045f23b8f222
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5260748
Reviewed-by: David Baron <dbaron@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1258910}