Implement ReactNativeDocument (#49012)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49012

Changelog: [internal]

(This is internal for now, until we rollout the DOM APIs in stable).

This refines the concept of root elements from the merged proposal for [DOM Traversal & Layout APIs](https://github.com/react-native-community/discussions-and-proposals/blob/main/proposals/0607-dom-traversal-and-layout-apis.md).

The original proposal included a reference to have the root node in the tree as `getRootNode()` and no other methods/accessors to access it.

This makes the following changes:
* The root node is a new abstraction in React Native implementing the concept of `Document` from Web. `node.getRootNode()`, as well as `node.ownerDocument` now return instances to this node (except when the node is detached, in which case `getRootNode` returns the node itself, aligning with the spec).
* The existing root node in the shadow tree is exposed as the `documentElement` of the new document instance. It would be the first and only child of the document instance, and the topmost parent of all the host nodes rendered in the tree.

In terms of APIs:
* Implements `getRootNode` correctly, according to the specified semantics.
* Adds `ownerDocument` to the `ReadOnlyNode` interface.
* Adds the `ReactNativeDocument` interface, which extends `ReadOnlyNode` (with no new methods on its own, which will be added in a following PR).

NOTE: This is currently gated under `ReactNativeFeatureFlags.enableDOMDocumentAPI` feature flag, which is disabled by default.

Reviewed By: yungsters

Differential Revision: D67526381

fbshipit-source-id: dff3645469e7ea2b2026dbbaa94d9fd0e00291be