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