[New Exercise] Add sgf-parsing exercise + New jsonString plugin to generator

[Sencudra]

1. Added new hard exercise
2. Added new plugin "jsonString" to the generator. Canonical data for this exercise includes JSON structures of potentially infinite depth. Existing Stencil rules and plugins cannot handle this. So, the JSON is converted into [String: Any] and then serialized into a string with appropriate escaping for insertion into tests.

[github-actions]

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[meatball133]

Looks good, as you may noticed so is the swift 6.0 ci broken. I would guess it has something to do with the underlying macos image has changed