src/vs/workbench/api/common/extHostCustomEditors.ts/ExtHostCustomEditors/supportEditing
class ExtHostCustomEditors:	private supportEditing(
		provider: vscode.CustomTextEditorProvider | vscode.CustomEditorProvider | vscode.CustomReadonlyEditorProvider
	): provider is vscode.CustomEditorProvider {
		return !!(provider as vscode.CustomEditorProvider).onDidChangeCustomDocument;
	}