jsonModify: allow positional insertion of array value?

[dansmith65]

• insert before other values
  • negative number?
  • "unshift" (like the json method) ?
• insert between values
  • decimal: 2.5 would insert after 2?
• insert at end
  • I assume this would be the most common use-case
  • empty keyOrIndex value?
  • this is how the function always operates at the time of creating this issue