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. std.json unittests fail with the version of std.json in master because of this.
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.

Tested this on both Windows and OSX.