The two aren't quite interchangeable: the Node definition has a few
more fields, which means that URLs created naively with `new URL()`
aren't assignable to that type.
Supplying a source map URL doesn't make sense for a file importer, for
two reasons:
1. A canonical `file:` URL is already the recommended source map URL
because it's always available on the local system.
2. Because a FileImporter doesn't know the fully-resolved canonical
URL of the file it loads, it doesn't have any way of determining
what the full source map URL should be anyway.
Because the doc comments for spec/js-api are written for
implementation authors, they aren't very useful as documentation for
end-users. This gives us a place to write user-facing documentation as
well as a test that verifies that the underlying type declarations are
kept in sync.
