Track in `geom_label_s()` and `geom_text_s()` the changes in 'ggplot2' 4.0.0

[aphalo]

See pull request in ggplot2 repo.
The approach curently in use in geom_label_s() in very similar but less flexible. Maintaining backwards compatibility is probably possible, forward compatibility with 'ggplot2' is even more important. I need to check this.

[aphalo]

ggplot2::geom_label() -> ggpp::geom_label_s()

• adopt support for defaults extracted from theme
• check consistency of already implemented support for line.width with the one added in 'ggplot2' 4.0.0
• can the new colour-targetting mechanism from 'ggplot2' 4.0.0 replace the one already in use in 'ggpp'? or be supported in parallel not to break existing code?

ggplot2::geom_text() -> ggpp::geom_text_s()

• adopt support for defaults extracted from theme