A few bug fixes (#2277)

* Comment tweak

* Directly print traceback

Since the shell.showtraceback is giving some issues

* Make InteracrtiveSceneEmbed into a class

This way it can keep track of it's internal shell; use of get_ipython has a finicky relationship with reloading.

* Move remaining checkpoint_paste logic into scene_embed.py

This involved making a few context managers for Scene: temp_record, temp_skip, temp_progress_bar, which seem useful in and of themselves.

* Change null key to be the empty string

* Ensure temporary svg paths for Text are deleted

* Remove unused dict_ops.py functions

* Remove break_into_partial_movies from file_writer configuration

* Rewrite guarantee_existence using Path

* Clean up SceneFileWriter

It had a number of vestigial functions no longer used, and some setup that could be made more organized.

* Remove --save_pngs CLI arg (which did nothing)

* Add --subdivide CLI arg

* Remove add_extension_if_not_present

* Remove get_sorted_integer_files

* Have find_file return Path

* Minor clean up

* Clean up num_tex_symbols

* Fix find_file

* Minor cleanup for extract_scene.py

* Add preview_frame_while_skipping option to scene config

* Use shell.showtraceback function

* Move keybindings to config, instead of in-place constants

* Replace DEGREES -> DEG

* Add arg to clear the cache

* Separate out full_tex_to_svg from tex_to_svg

And only cache to disk the results of full_tex_to_svg.  Otherwise, making edits to the tex_templates would not show up without clearing the cache.

* Bug fix in handling BlankScene

* Make checkpoint_states an instance variable of CheckpointManager

As per https://github.com/3b1b/manim/issues/2272

* Move resizing out of Window.focus, and into Window.init_for_scene

* Make default output directory "." instead of ""

To address https://github.com/3b1b/manim/issues/2261

* Remove input_file_path arg from SceneFileWriter

* Use Dict syntax in place of dict for config more consistently across config.py

* Simplify get_output_directory

* Swap order of preamble and additional preamble