[JS] Forbid export of interfaces

With the only exception of external interfaces.

See https://youtrack.jetbrains.com/issue/KT-44099

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

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