Fix toggling debug info crashing the editor

[knopp]

Fixes https://github.com/superlistapp/super_editor/issues/2254

• Changes Stack in SuperEditorImeDebugVisuals to SliverHybridStack because the render object now lives in a sliver
• Implements forgetChild inside ContentLayersElement. This is necessary because the content of ContentLayersElement is being reparented using GlobalKey so it needs to be excluded from visitChildren when unmounting.