Add 'default' keyword

Parser: allow objects that are marked as default to have no name

compiler/testData/psi/DefaultKeyword.kt

@@ -0,0 +1,85 @@
+class A {
+    default object {
+
+    }
+}
+
+class A {
+    default object
+
+    val c: Int = 1
+}
+
+class B {
+    public default object A {
+
+    }
+}
+
+class B {
+    default object A {
+        default object {
+        }
+    }
+}
+
+default object B
+//should be error
+default object {
+
+}
+
+object A {
+    default object
+}
+
+trait A {
+    default object
+
+    class C {
+        default object C {
+            default object
+        }
+    }
+}
+
+enum class D {
+    A B
+
+    default object
+}
+
+class A {
+    default class object
+}
+
+//should be error
+class A {
+    class default object
+}
+
+class A {
+    default public final object
+}
+
+//should be error
+default class {}
+
+//should be error
+val t = default object {
+
+}
+
+enum class I {
+    A
+    B
+
+    default object
+}
+
+enum class I {
+    A
+    B
+
+    default object {}
+}