Replace the 'format(...)' Metadata with 'sliced-format'

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Slice currently supports 3 different arguments for the format metadata: default, compact, and sliced. In 4.0 compact will become the new default, cutting this list down to 2 options. Having a metadata directive for just 2 options seems excessive, especially when one of them is already set by default.

Instead, we should remove the metadata, and add a new sliced-format metadata. When it's specified, the format will be sliced, and when it isn't, it will be compact by default.

[InsertCreativityHere]

See related todo: https://github.com/zeroc-ice/ice/blob/9b86f942d90a3cd3ed4440d3cfcf5d810841f096/cpp/src/Slice/Parser.cpp#L992-L994