Fix issue 20511: make toJSON determine safeness based on output range safeness.

[FeepingCreature]

Works around DMD being unable to infer the recursive toJson call as @safe.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20511	regression	Can't format JSONValue to OutputRange due to @safe

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "stable + phobos#7354"

[FeepingCreature]

Pretty sure the auto-tester failure is sporadic.