Fixes #93219

#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

For #77131

- Use a class for `CustomDocument` instead of an interface. Extensions can now add their own data to a `CustomDocument` by sublassing

- Renamed `resolveCustomDocument` to `openCustomDocument` and require that extensions return a `CustomDocument`

- Exposed edits on `CustomDocument`

- Made the third parameter of `registerCustomEditorProvider` a generic options bag that takes a `webviewOptions`

https://github.com/microsoft/vscode/commit/789416f4a7f5fa42acbeb9d4df9907255d1d0f80#commitcomment-37996419

Fixup progress, execution