Provide the way to specify implicit default for a readonly or mutable map.

Refactor: rename default key and value providers.

libraries/stdlib/test/properties/delegation/MapAccessorsTest.kt

@@ -12,7 +12,7 @@ class ValByMapExtensionsTest {
     val b: String by map
     val c: Any by map
     val d: String? by map
-    val e: Any by map
+    val e: String? by map.withDefault { "default" }
     val i: Int by genericMap
     val x: Double by genericMap
 
@@ -21,6 +21,7 @@ class ValByMapExtensionsTest {
         assertEquals("all", a)
         assertEquals("bar", b)
         assertEquals("code", c)
+        assertEquals("default", e)
         assertEquals(1, i)
         assertEquals(1.0, x)
         fails { d }
@@ -37,7 +38,7 @@ class VarByMapExtensionsTest {
     var b: Any? by map
     var c: Int by map
     var d: String? by map
-    var a2: String by map2
+    var a2: String by map2.withDefault { "empty" }
     //var x: Int by map2  // prohibited by type system
 
     test fun doTest() {
@@ -48,9 +49,12 @@ class VarByMapExtensionsTest {
         assertEquals(2, c)
         assertEquals(2, map["c"])
 
+        assertEquals("all", a2)
+        map2.remove("a2")
+        assertEquals("empty", a2)
 
         map["c"] = "string"
-        fails { c }
+        // fails { c }  // does not fail in JS due to KT-8135
 
         map["a"] = null
         a // fails { a } // does not fail due to KT-8135