Add support for JSON type Index

[Guy Korland]

Two more features were added specific to JSON:

On FT.CREATE:

• ON JSON

• identifier AS Attribute
  e.g.
  FT.CREATE idx ON JSON PREFIX 1 doc: SCHEMA $.product.label AS label TEXT SORTABLE $.product.stock AS stock NUMERIC SORTABLE

On FT.SEARCH:

• RETURN {jsonPath} AS {property}
  e.g.
  FT.SEARCH products * RETURN 2 $.product.label AS label $.reviews.content AS reviews