Exploring new geoms for diverging plots

[larmarange]

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• [x] renaming position_likert_count() into position_diverging()
• [x] add an option default_by in stat_ptop()
• [x] geom_bar_diverging(), geom_bar_likert() and geom_bar_pyramid()
• [x] geom_text_diverging(), geom_text_likert() and geom_text_pyramid()
• [x] geom_bar_prop() and geom_text_prop()
• [ ] improve/clean doc and testing
• [ ] add a vignette