refactor!: Use navigation rulesets instead of ASTNode to control keyboard navigation. (#8992)

* feat: Add interfaces for keyboard navigation.

* feat: Add the Navigator.

* feat: Make core types conform to INavigable.

* feat: Require FlyoutItems elements to be INavigable.

* feat: Add navigation policies for built-in types.

* refactor: Convert Marker and LineCursor to operate on INavigables instead of ASTNodes.

* chore: Delete dead code in ASTNode.

* fix: Fix the tests.

* chore: Assuage the linter.

* fix: Fix advanced build/tests.

* chore: Restore ASTNode tests.

* refactor: Move isNavigable() validation into Navigator.

* refactor: Exercise navigation instead of ASTNode.

* chore: Rename astnode_test.js to navigation_test.js.

* chore: Enable the navigation tests.

* fix: Fix bug when retrieving the first child of an empty workspace.

core/field_number.ts

@@ -341,6 +341,18 @@ export class FieldNumber extends FieldInput<number> {
       options,
     );
   }
+
+  /**
+   * Returns this field's class.
+   *
+   * Used by keyboard navigation to look up the rules for navigating from this
+   * field.
+   *
+   * @returns This field's class.
+   */
+  getClass() {
+    return FieldNumber;
+  }
 }
 
 fieldRegistry.register('field_number', FieldNumber);