Fix ordering of the properties when serializing

It looks like the order of the properties when we serialize some structure in JSON-LD is important.

It hinders the performances on Comunica side if we wish to explore the LDFlex integration.

See for reference:

• https://github.com/comunica/comunica/issues/772#issuecomment-758572710
• https://www.w3.org/TR/json-ld11-streaming/#streaming-document-form

Looks like the order of the most important properties are:

{
   "@context": "",
   "@type": "",
   "@id": "" 
   .....
}