src/vs/workbench/api/common/extHostCustomEditors.ts/isCustomTextEditorProvider
function isCustomTextEditorProvider(provider: vscode.CustomReadonlyEditorProvider<vscode.CustomDocument> | vscode.CustomTextEditorProvider): provider is vscode.CustomTextEditorProvider {
	return typeof (provider as vscode.CustomTextEditorProvider).resolveCustomTextEditor === 'function';
}