[DTD] Add a `getActiveLocation` method to the `Editor` service

[bkonyi]

While it's currently possible to listen to the DTD Editor to respond to changes in the currently selected file in the IDE, there's no way to query the current state manually.

The implementers of the Editor service should add support for a ActiveLocation getActiveLocation() method which returns the currently selected location.

ActiveLocation would have roughly the following interface:

class ActiveLocation {
  /// An identifier that represents the active document.
  ///
  /// `null` is there is no active document.
  OptionalVersionedTextDocumentIdentifier? textDocument;

  /// The set of selections in the document.
  ///
  /// There will always be at least one selection if there is an active document
  /// (textDocument != null), but there may also be more if a user has multiple
  /// selections active.
  ///
  /// The first selection is always the primary selection for actions that only
  /// support one location.
  List<EditorSelection> selections;
}

The interface for ActiveLocationChangedEvent would now effectively be:

class ActiveLocationChangedEvent extends ActiveLocation implements Event {
  String eventKind;
}

[DanTup]

Sounds good to me, with one tweak:

ActiveLocation getActiveLocation();

The return value should be nullable, because there might not be an active file.

@bkonyi were you planning on adding to https://github.com/dart-lang/sdk/blob/main/pkg/dtd_impl/dtd_common_services_editor.md or do you want me to? (We should probably commit it there and ensure we're all agreed before adding to editors).