fix(rrweb-snapshot): inserting doctype element

[JonaszJestem]

In some rare cases I've noticed that during replay I do get an error in this specific case. It fails with:

HierarchyRequestError: Failed to execute 'appendChild' on 'Node': Can't insert a doctype after the root element
  at buildNodeWithSN
  at rebuild
  at Ya.rebuildFullSnapshot
  at castFn
  at wrappedCastFn
  at Ya.<anonymous>
  at play

[changeset-bot]

🦋 Changeset detected

Latest commit: 9c2bcb4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 19 packages

Name	Type
rrweb-snapshot	Patch
rrweb	Patch
rrdom	Patch
rrdom-nodejs	Patch
rrweb-player	Patch
@rrweb/all	Patch
@rrweb/replay	Patch
@rrweb/record	Patch
@rrweb/types	Patch
@rrweb/packer	Patch
@rrweb/utils	Patch
@rrweb/web-extension	Patch
rrvideo	Patch
@rrweb/rrweb-plugin-console-record	Patch
@rrweb/rrweb-plugin-console-replay	Patch
@rrweb/rrweb-plugin-sequential-id-record	Patch
@rrweb/rrweb-plugin-sequential-id-replay	Patch
@rrweb/rrweb-plugin-canvas-webrtc-record	Patch
@rrweb/rrweb-plugin-canvas-webrtc-replay	Patch

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[eoghanmurray]

The test case you have added has the doctype out of order. Was this a real world example, or is this a synthetic example to trigger the case?
I'm asking as it might be something we should also be fixing at record time if snapshot.ts is generating things in an out of order way.
Other possibility is that you are experiencing doctype added by a mutation?