Move the memory usage status bar item to the nbresuse repo

[jtpio]

In #9349, we realize that it would be simpler for the memory usage status bar item to be moved to the nbresuse repo, and developed as a third-party extension.

This item currently lives in lab here:

jupyterlab/packages/statusbar-extension/src/index.ts

Lines 327 to 356 in ae08605

	/**
	* A plugin providing memory usage statistics to the application.
	*
	* #### Notes
	* This plugin will not work unless the memory usage server extension
	* is installed.
	*/
	export const memoryUsageItem: JupyterFrontEndPlugin<void> = {
	id: '@jupyterlab/statusbar-extension:memory-usage-status',
	autoStart: true,
	requires: [IStatusBar, ITranslator],
	activate: (
	app: JupyterFrontEnd,
	statusBar: IStatusBar,
	translator: ITranslator
	) => {
	const item = new MemoryUsage(translator);
	statusBar.registerStatusItem(
	'@jupyterlab/statusbar-extension:memory-usage-status',
	{
	item,
	align: 'left',
	rank: 2,
	isActive: () => item.model!.metricsAvailable,
	activeStateChanged: item.model!.stateChanged
	}
	);
	}
	};

This would make it easier for nbresuse to move independently from JupyterLab, and avoid syncing on both repos for major releases.

The extension can be developed as federated / pre-built extension, using the new distribution system with pip and conda:

https://jupyterlab.readthedocs.io/en/latest/developer/extension_dev.html#goals-of-the-federated-extension-system

Which means that the extension would be automatically installed on pip install nbresuse, and would be seamless to the users since they already have to perform this step anyway.