[Issue 19256] std.json: JSONValue allows violating constness

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19256

--- Comment #5 from FeepingCreature <default_357-line at yahoo.de> ---
Fixed by https://github.com/dlang/dmd/pull/16594 when
`-preview=fixImmutableConv` is enabled.

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