[K/JS] Add ability to exclude declarations from export by a new annotation @JsExport.Ignore.

compiler/testData/codegen/box/multiplatform/optionalExpectation.kt

@@ -13,7 +13,10 @@ package a
 expect annotation class A(val x: Int)
 
 @OptionalExpectation
-expect annotation class B(val s: String)
+expect annotation class B(val s: String) {
+    @OptionalExpectation
+    annotation class C(val a: Boolean)
+}
 
 // FILE: actual.kt
 
@@ -33,6 +36,7 @@ import a.B
 
 @A(42)
 @B("OK")
+@B.C(true)
 fun box(): String {
     return "OK"
 }

compiler/testData/multiplatform/optionalExpectationIncorrectUse/common.kt

@@ -20,7 +20,6 @@ annotation class InOtherAnnotation(val a: A)
 fun useInOtherAnnotation() {}
 
 
-
 expect class C {
     @OptionalExpectation
     annotation class Nested

compiler/testData/multiplatform/optionalExpectationIncorrectUse/output.txt

@@ -16,9 +16,6 @@ annotation class InOtherAnnotation(val a: A)
 compiler/testData/multiplatform/optionalExpectationIncorrectUse/common.kt:19:20: error: declaration annotated with '@OptionalExpectation' can only be used inside an annotation entry
 @InOtherAnnotation(A())
                    ^
-compiler/testData/multiplatform/optionalExpectationIncorrectUse/common.kt:25:5: error: '@OptionalExpectation' cannot be used on a nested class
-    @OptionalExpectation
-    ^
 
 -- JVM --
 Exit code: COMPILATION_ERROR
@@ -41,9 +38,6 @@ annotation class InOtherAnnotation(val a: A)
 compiler/testData/multiplatform/optionalExpectationIncorrectUse/common.kt:19:20: error: declaration annotated with '@OptionalExpectation' can only be used inside an annotation entry
 @InOtherAnnotation(A())
                    ^
-compiler/testData/multiplatform/optionalExpectationIncorrectUse/common.kt:25:5: error: '@OptionalExpectation' cannot be used on a nested class
-    @OptionalExpectation
-    ^
 compiler/testData/multiplatform/optionalExpectationIncorrectUse/jvm.kt:1:24: error: declaration annotated with '@OptionalExpectation' can only be used inside an annotation entry
 fun useInReturnType(): A? = null
                        ^