[FIR2IR] Make convertAndActualize method the only entrypoint to fir2ir

Fir2Ir conversion consists of multiple steps with complex logic (like
  conversion of each module, actualization, plugins application, constant
  evaluation), and to ensure that they all are executed correctly it's
  convenient to have the single entry point for all this machinery

js/js.tests/test/org/jetbrains/kotlin/js/test/converters/FirJsKlibBackendFacade.kt

@@ -74,7 +74,7 @@ class FirJsKlibBackendFacade(
                 abiVersion = KotlinAbiVersion.CURRENT, // TODO get from test file data
                 jsOutputName = null
             ) {
-                inputArtifact.serializeSingleFile(it, TODO("Will be fixed in the next commit"))
+                inputArtifact.serializeSingleFile(it)
             }
         }
 

js/js.tests/test/org/jetbrains/kotlin/js/test/converters/JsKlibBackendFacade.kt

@@ -63,7 +63,7 @@ class JsKlibBackendFacade(
                 abiVersion = KotlinAbiVersion.CURRENT, // TODO get from test file data
                 jsOutputName = null
             ) {
-                inputArtifact.serializeSingleFile(it, TODO("Will be fixed in the next commit"))
+                inputArtifact.serializeSingleFile(it)
             }
         }