New custom editor API proposal

For #77131

Fixes #93963
Fixes #94515
Fixes #94517
Fixes #94527
Fixes #94509
Fixes #94514
Fixes #93996
Fixes #93913

This removes explicit edits from the API and reshapes the API to more closely match VS Code's internal API. The change also tries to better express the lifecycle of backups

extensions/markdown-language-features/src/features/previewManager.ts

@@ -151,7 +151,7 @@ export class MarkdownPreviewManager extends Disposable implements vscode.Webview
 	}
 
 	public async openCustomDocument(uri: vscode.Uri) {
-		return new vscode.CustomDocument(uri);
+		return { uri, dispose: () => { } };
 	}
 
 	public async resolveCustomTextEditor(