Formatter: support trailing comma in when entry

#KT-34744

idea/testData/formatter/trailingComma/whenEntry/WhenEntry.after.kt

@@ -0,0 +1,69 @@
+fun foo(x: Any) = when (x) {
+    Comparable::class, Iterable::class, String::class, // trailing comma
+    -> println(1)
+    else -> println(3)
+}
+
+fun foo(x: Any) {
+    when (x) {
+        Comparable::class, Iterable::class, String::class, /*// trailing comma*/
+        -> println(1)
+        else -> println(3)
+    }
+
+    when (x) {
+        Comparable::class, Iterable::class, String::class /*// trailing comma*/ -> println(1)
+        else -> println(3)
+    }
+
+    when (x) {
+        1 -> {
+
+        }
+        else -> println(3)
+    }
+
+    when (x) {
+        1,
+        -> {
+
+        }
+        else -> println(3)
+    }
+
+    when (x) {
+        1,
+        -> {
+
+        }
+        else -> println(3)
+    }
+
+    when (x) {
+        1, 2,
+        3, /**/
+        -> {
+
+        }
+        else -> println(3)
+    }
+
+    when (val c = x) {
+        1, 2,
+        3, /**/
+        -> {
+
+        }
+        else -> println(3)
+    }
+
+    when {
+        x in coll,
+        -> {
+
+        }
+        else -> println(3)
+    }
+}
+
+// SET_TRUE: ALLOW_TRAILING_COMMA