Add "sorted" to JSONOptions

[dlangBugzillaToGithub]

andre reported this on 2018-11-10T11:52:51Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=19390

Description

While calling std.json: toPrettyString I want to have an option to get the keys sorted. An option "sorted" could be added to JSONOptions.

Use case: I automatic create and update a JSON file. A user also updates this file manually. To have a stable order of the keys, the keys should always be stored in a sorted way.
This is good for the user and also for git diffs.