Issue 13013 - Converted real to double in std.json.JSONValue

https://issues.dlang.org/show_bug.cgi?id=13013

1) When compiling with -m64 on Windows (when using the MSVC linker), the real type is unsupported.
2) JSON only uses 64-bit floating point

=> Changed real to double, as it is more functional and no less correct.

One concern: ABI compatibility.