Fix JsonEncode treating tables with index 0 as arrays

[Kipstz]

Summary

• Fix Util.JsonEncode incorrectly encoding Lua tables with index 0 as JSON arrays
• Added IsLuaArray() helper function that validates all keys are integers >= 1
• Tables with non-standard indices (like 0) are now correctly encoded as JSON objects

Closes #348

Before

Util.JsonEncode({[0]="a", [1]=true, [2]=0.1})
-- Output: ["a",true,0.1]  (incorrect - data loss)

After

Util.JsonEncode({[0]="a", [1]=true, [2]=0.1})
-- Output: {"0":"a","1":true,"2":0.1}  (correct)

[WiserTixx]

The issue also mentions this related behaviour:
#348 (comment)

I'd like to add another weird behavior with tables to the ticket, with Util.JsonDecode this time:

When I run Util.JsonDecode('[1,null,3,4]') I expect to get this object : { [1] = 1, [3] = 3, [4] = 4 }

But instead I get : { [1] = 1, [2] = 3, [3] = 4 } or simplified { 1, 3, 4 }

The null 2nd index was completely omitted by the JSON parser

Would you mind attempting to fix this as well in this pr?

[Kipstz]

The issue also mentions this related behaviour: #348 (comment)

I'd like to add another weird behavior with tables to the ticket, with Util.JsonDecode this time:
When I run Util.JsonDecode('[1,null,3,4]') I expect to get this object : { [1] = 1, [3] = 3, [4] = 4 }
But instead I get : { [1] = 1, [2] = 3, [3] = 4 } or simplified { 1, 3, 4 }
The null 2nd index was completely omitted by the JSON parser

Would you mind attempting to fix this as well in this pr?

Yes, I can. I'll look at this tomorrow.