Split JS conversions from ScriptInterface.

All ToJSVal/FromJSVal definitions are put in a separate header.
Remove AssignOr[To/From]JSVal duplication.

The functions were already static so this is rather straightforward.

Follows 34b1920e7b and 2bae30c454

Differential Revision: https://code.wildfiregames.com/D3953
This was SVN commit r25428.

source/graphics/MapGenerator.cpp

@@ -323,7 +323,7 @@ JS::Value CMapGeneratorWorker::LoadHeightmap(const VfsPath& filename)
 
 	ScriptRequest rq(m_ScriptInterface);
 	JS::RootedValue returnValue(rq.cx);
-	ToJSVal_vector(rq, &returnValue, heightmap);
+	Script::ToJSVal(rq, &returnValue, heightmap);
 	return returnValue;
 }