#77131

Adds custom editor restoring to the backup restorer. This PR includes:

- Adds a `vscode-custom-editor` scheme that we use internally for custom editors. This ensures that each custom editor has it's own editing history (even if another custom editor opens the same resource).

- Make the backup restorer understand how to restore  `vscode-custom-editor` editors. This is done by adding a `IEditorInputFactoryRegistry` that lets the custom editor hook into the backup restorer