JS backend: fix KT-6112 jsonFromTuples is broken

#KT-6112 Fixed

js/js.libraries/src/core/json.kt

@@ -5,11 +5,13 @@ native public class Json() {
     public fun set(propertyName: String, value: Any?): Unit = noImpl
 }
 
-library("jsonFromTuples")
-public fun json(vararg pairs: Pair<String, Any?>): Json = noImpl
-
-library("jsonFromTuples")
-public fun json2(pairs: Array<Pair<String, Any?>>): Json = noImpl
+public fun json(vararg pairs: Pair<String, Any?>): Json {
+    var res: dynamic = js("({})")
+    for((name, value) in pairs) {
+        res[name] = value
+    }
+    return res
+}
 
 library("jsonAddProperties")
 public fun Json.add(other: Json): Json = noImpl

js/js.libraries/test/core/JsonTest.kt

@@ -0,0 +1,14 @@
+package test.js
+
+import kotlin.test.*
+import org.junit.Test as test
+
+class JsonTest {
+
+    test fun createJsonFromPairs() {
+        var obj = json(Pair("firstName", "John"), Pair("lastName", "Doe"), Pair("age", 30))
+        assertEquals("John", obj["firstName"], "firstName")
+        assertEquals("Doe", obj["lastName"], "lastName")
+        assertEquals(30, obj["age"], "age")
+    }
+}