[JS] JsExport diagnostics and legacy support

Account for JsExport in legacy backend namer. It means we
catch overloaded exported function conflicts for free!

Add error diagnostics:
* NESTED_JS_EXPORT (Fixes KT-36798)
* WRONG_EXPORTED_DECLARATION (Part of the fix for KT-37752)
* NON_EXPORTABLE_TYPE (Fixes KT-37771)

js/js.translator/testData/box/char/topLevelCallables.kt

@@ -1,16 +1,15 @@
 // SKIP_MINIFICATION
+
+// Exported declaration uses non-exportable return type: Char
 // IGNORE_BACKEND: JS_IR
+
 @JsName("foo")
-@JsExport
 fun foo(): Char = '1'
 
-@JsExport
 val p1: Char = '2'
 
-@JsExport
 var p2: Char = '3'
 
-@JsExport
 var p3: Char = '4'
     get() = field + 1
     set(value) {

js/js.translator/testData/box/coercion/charValParameter.kt

@@ -1,7 +1,6 @@
 // EXPECTED_REACHABLE_NODES: 1276
 // IGNORE_BACKEND: JS_IR
 
-@JsExport
 class A(val x: Char)
 
 fun typeOf(x: dynamic): String = js("typeof x")

js/js.translator/testData/box/coercion/defaultAccessors.kt

@@ -4,7 +4,6 @@ interface I {
     val a: Char
 }
 
-@JsExport
 object X : I {
     override var a = '#'
 }

js/js.translator/testData/box/coercion/propertyBridgeChar.kt

@@ -1,7 +1,6 @@
 // EXPECTED_REACHABLE_NODES: 1289
 // IGNORE_BACKEND: JS_IR
 
-@JsExport
 open class A {
     val foo: Char
         get() = 'X'

js/js.translator/testData/box/inline/inlinedObjectLiteralIsCheck.kt

@@ -1,5 +1,6 @@
 // EXPECTED_REACHABLE_NODES: 1282
 
+@JsExport
 interface I {
     fun ok(): String
 }

js/js.translator/testData/box/propertyAccess/accessorsWithJsName.kt

@@ -12,7 +12,6 @@ class A {
         }
 }
 
-@JsExport
 val A.z: Int
     @JsName("getZ_") get() = 42