Feature Request: Make options_description output optionally machine readable (ie --helpjson) to support auto-completion of process *arguments* in system shells

This is a dupe of a feature request I filed for Qt's QCommandLineParser, so I'll just link to that instead of copy/pasting the description here: https://bugreports.qt.io/browse/QTBUG-74594

Start with the linked Qt request, QTBUG-74594, since the issue refers to it for the feature requirements. Then inspect options_description output and define what a machine-readable --helpjson format would need to expose for shell completion of process arguments; done means that output is available optionally without changing the existing human-readable help.

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