[FEATURE REQUEST] Implement `ChildNode.before()` and `ChildNode.after()`

In accordance with https://github.com/ampproject/worker-dom/blob/main/web_compat_table.md, I'd like to request the DOM APIs `ChildNode.before()` and `ChildNode.after()`.

# Use case

I intend to use this to lazy-load sections of a single-page website. Once a section finishes loading, it's neighboring sections (top and bottom) will be fetched, then to add them to the DOM, I intend to use `before()` or `after()` on the previous section.

Start with web_compat_table.md and compare the requested ChildNode.before() and ChildNode.after() APIs with the project's existing DOM compatibility work. Trace the relevant TypeScript DOM implementation entry points, then verify that both methods support inserting neighboring sections as described in the use case.

Written by the indexing model from the issue text.