Optionally allow stringification of numeric keys in tables when calling `lljson.encode()`

[canny]

I was trying to encode a table for debugging and it was just

{ [50] = true }

Maybe an object meta table would be useful?

lljson.object_mt

To mark a sparse array to be encoded as an object instead?

In the end i got around it using slencode but it might be useful to be able to output normal json with number keys, as a json object.

https://secondlife.canny.io/admin/board/slua-alpha/p/optionally-allow-stringification-of-numeric-keys-in-tables-when-calling-lljsonen

[canny]

This issue has been linked to a Canny post: Optionally allow stringification of numeric keys in tables when calling lljson.encode() 🎉