refactor: Remove INavigable in favor of IFocusableNode. (#9037)

* refactor: Remove INavigable in favor of IFocusableNode.

* chore: Fix JSDoc.

* chore: Address review feedback.

tests/typescript/src/field/different_user_input.ts

@@ -61,10 +61,6 @@ class FieldMitosis extends Field<CellGroup> {
     });
     this.value_ = {cells};
   }
-
-  getClass() {
-    return FieldMitosis;
-  }
 }
 
 fieldRegistry.register('field_mitosis', FieldMitosis);