JS: support SAM conversion

The SAM adapter is generate on declaration site. This is different
from the JVM approach.

`external fun interface` is banned for now.

Reusing interface declaration for the adapter is a hack which
reduces code size and makes importing/exporting the adapter
effortless.

compiler/testData/codegen/box/funInterface/basicFunInterfaceConversion.kt

@@ -1,6 +1,5 @@
 // !LANGUAGE: +NewInference +FunctionalInterfaceConversion +SamConversionPerArgument +SamConversionForKotlinFunctions
 // IGNORE_BACKEND_FIR: JVM_IR
-// IGNORE_BACKEND: JS
 // SKIP_DCE_DRIVEN
 
 fun interface Foo {