add 2908.kt,10821.kt & complete all in zeta_kotlin

zeta_kotlin/completed/10821.kt

@@ -0,0 +1,3 @@
+fun main() {
+    println(readLine()?.split(",")?.size)
+}

zeta_kotlin/completed/2908.kt

@@ -0,0 +1,7 @@
+fun main(){
+    val (a, b) = readLine()!!.split(" ")
+    val a_ = a.reversed().toInt()
+    val b_ = b.reversed().toInt()
+    
+    if (a_ > b_) {println(a_)} else {println(b_)}
+}