JS: prohibit non-abstract members of external interfaces, except for nullable properties. See KT-15308

compiler/testData/diagnostics/testsWithJsStdLib/native/optionlBody/native.kt

@@ -11,7 +11,7 @@ external interface T {
     val baz: Int
 
     fun foo()
-    fun bar() {}
+    fun bar()
 
     companion object {
         val baz: Int

compiler/testData/diagnostics/testsWithJsStdLib/native/optionlBody/native.txt

@@ -40,7 +40,7 @@ public external object O {
 
 public external interface T {
     public abstract val baz: kotlin.Int
-    public open fun bar(): kotlin.Unit
+    public abstract fun bar(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public abstract fun foo(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int

compiler/testData/diagnostics/testsWithJsStdLib/native/optionlBody/nativeGetter.kt

@@ -9,5 +9,5 @@ external interface T {
     @nativeGetter
     fun foo(d: Double): String?
     @nativeGetter
-    fun bar(d: Double): String? = noImpl
+    fun bar(d: Double): String?
 }

compiler/testData/diagnostics/testsWithJsStdLib/native/optionlBody/nativeGetter.txt

@@ -4,7 +4,7 @@ package
 @kotlin.js.nativeGetter public fun kotlin.String.foo(/*0*/ n: kotlin.Int): kotlin.Int?
 
 public external interface T {
-    @kotlin.js.nativeGetter public open fun bar(/*0*/ d: kotlin.Double): kotlin.String?
+    @kotlin.js.nativeGetter public abstract fun bar(/*0*/ d: kotlin.Double): kotlin.String?
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     @kotlin.js.nativeGetter public abstract fun foo(/*0*/ d: kotlin.Double): kotlin.String?
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int