* 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.
