add support for nested shadow dom

[YunFeng0817]

scenario

If a page highly depends on custom elements, there may be lots of nested shadow dom. #754 may as an example
And the current version doesn't support recording shadow doms which were attached after the full snapshot.

what changed

1. Hijack HTMLElement.attachShadow. When we attach a shadow dom on a created element, the old version can't detect the change and the new shadow dom can't be observed.
2. If we attach a shadow dom on a created element, the serialized element data won't contain 'isShadowHost' property. So I have to add a workaround on the replay side.
3. notInDoc check doesn't work for nested shadow dom so I changed the checking way.
4. add recording test for nested shadow dom

Related issues: #38, #754

[Juice10]

I'm new to shadowDom, so probably not the best to judge this, but the PR looks good to me!

[Yuyz0112]

Looks this is the only way to observe the attaching behavior.