Reset state upon manual terminal closing

[Splines]

With this PR, we detect the event of a user manually closing a terminal that hosts the active Manim IPython session. Further behavior is described in the docstrings:

manim-notebook/src/manimShell.ts

Lines 644 to 655 in 8283ad0

	/**
	* This event is fired when a terminal is closed manually by the user.
	* In this case, we can only do our best to clean up since the terminal
	* is probably not able to receive commands anymore. It's mostly for us
	* to reset all states such that we're ready for the next command.
	*
	* When closing while previewing a scene, the terminal is closed, however
	* we are not able to send a keyboard interrupt anymore. Therefore,
	* ManimGL will take a few seconds to actually close its window. When
	* the user employs our "Quit preview" command instead, the preview will
	* be closed immediately.
	*/

Some reset events might be fired multiple times in the end, but it's better to be safe than sorry here. Multiple calls to resetActiveShell() from different places of the code is not a problem.

While testing this, please try to force-quit the terminal manually in many different situations to make sure that no state is left dangling, e.g. some notifications still showing or subsequent commands not working anymore.

[VladimirFokow]

works for me
(tested several edge cases, didn't find problems)