Fixes #88229

There should a 1:1 mapping between VS Code's custom editors and custom editors (specifically the webviews) that extensions know about. This was previously not true as a single `CustomEditorInput` could end up being shared between multiple editor groups.

With this change, we always create a copy of the custom editor input during split, and then add a listener for `onDidCloseEditor` so that we can make sure we get rid of the unique inputs (and notify extensions of this)

So far, file editors have reported isDirty() === false when auto save was enabled e.g. to avoid showing dirty indicators. With the advent of custom editors, this needs to be cleaned up. The new method isSaving() serves as a hint that an editor can be dirty but in the process of being saved. As such, we can decide to hide the dirty indicator in that case. But in other cases we may want to know if the editor is dirty or not, even if auto saved.