Feature: make `pillow` optional dependency

[mokurin000]

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Feature Details

I believe Pillow is only used for pysimplegui backend, which is planned to deprecate in Cli2Gui (#21)

Describe the solution you'd like

move pillow to extra group pysimplegui e.g.

Describe alternatives you've considered

If you've thought of other ways to address the issue or implement the feature, please describe those alternatives here.

Additional Context

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