Reload user-defined modules during `reload()` (#2257)

* Experiment a lot with module loading

* Extract methods out of experimental mess

* Fix get module return type

* Only reload() modules during reload() command

* Remove unnecessary default parameter

* Add docstrings and logging statements

* Delete unwanted printout

* Improve logging messages

* Extract methods to a new class ModuleLoader

* Remove unused builtins import

* exec_module in any case at the end

* Clarify docstrings & move get_module method up in file

* Add more additionally excluded modules as array

* Distinguish between user-defined modules and external libraries like numpy

* Improved tracked_import docstring

* Remove _insert_embed suffix before logging

* Fix args.is_reload not defined error

* Refine logic to determine whether module is user-defined or not

* Fix list vs. set type annotations

* Improve docstrings & change order of early return

* Fix spelling mistake of "Reloading"

* Try out custom deep reload

* Make deep reload more robust

* Also reload modules imported as classes

* Move early return up to greatly improve performance

* Clean up comments

* Make methods of Module Loader "private"

* Add backticks around function in docstring

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Co-authored-by: Grant Sanderson <grant@3blue1brown.com>