src/vs/workbench/services/language/common/languageService.ts/isUndefinedOrStringArray
function isUndefinedOrStringArray(value: string[]): boolean {
	if (typeof value === 'undefined') {
		return true;
	}
	if (!Array.isArray(value)) {
		return false;
	}
	return value.every(item => typeof item === 'string');
}