Need to update mitigation of file editors extensionFactory issue before release

[krassowski]

In #1057 a workaround calling the editorAdded signal was added:

jupyterlab-lsp/packages/jupyterlab-lsp/src/adapters/fileeditor.ts

Lines 33 to 41 in 376c996

	protected async initOnceReady(): Promise<void> {
	await super.initOnceReady();
	// Workaround for
	// https://github.com/jupyterlab/jupyterlab/issues/15868
	// remove once fixed upstream
	this._editorAdded.emit({
	editor: this.activeEditor
	});
	}

but once this is emitted upstream after jupyterlab/jupyterlab#15868 gets fixed, we could be in an incorrect state of two copies of each CM6 extension getting added to the editor.

We need to either:

• wait with a release for an upstream fix to land
• devise a way to make this workaround only apply when upstream does not handle it on it's own