Add group function (#3175)

src/prelude/string.ts

@@ -1,3 +1,7 @@
+export function concat(xs: string[]): string {
+	return xs.reduce((a, b) => a + b, "");
+}
+
 export function capitalize(s: string): string {
 	return toUpperCase(s.charAt(0)) + toLowerCase(s.slice(1));
 }