* Introduce branching sections
* parent_index -> branch_parent_index
* Flip the branch icon
* Don't mark branching sections as aborted if the main flow crashes
* Outline more details about branching sections in the Elixir and Livebook notebook
* Add branch indicator to the sections sidebar
* Add support for Mix runtime as a default
* Support default runtime options in the CLI
* Set cell status to queued while runtime is being started
* Clean up tests