Update JSON.ahk

Get the **character code** in hex derectly! Only need to replace "0x" to "\u".
Much faster I think!
Test:
```ahk
s:=0x12AB
SetFormat, IntegerFast, hex
b:=A_TickCount
	Loop 1000000{
		;~ y:="\u",n := 12
		;~ while(n>=0)
			;~ y.= Chr((x:=(s>>n) & 15) + (x<10 ? 48 : 55))
			;~ ,n-=4
		;10438 ms

		y:="\u" SubStr(s,3)
		;812
	}
MsgBox % A_TickCount-b
```

I don't know, how to get NumberType of SetFormat for restore later?
I will ask in fourm.