CLI custom commands for 4.2

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In 4.2, there's a new option to add custom CLI commands to Blender via #119115 CLI: support defining custom commands via C++ & Python API. May be a great new alternative to running BLint offline.

Once 4.2 is officially released, we'll need to add this without breaking backwards compatibility.

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Updated in docs: docs.blender.org/api/current/bpy.utils.html