Export `alpha` argument in `element_rect()`

[Yunuuuu]

I would like to request the addition of an alpha argument to the element_rect() function in ggplot2. Adding an alpha argument to element_rect() would make it easier for users to adjust the transparency of rectangular elements without needing to manually calling ggplot2::alpha() or ggplot2::fill_alpha()

[teunbrand]

For some more context: #6249 (comment)

I don't have any strong opinion on whether this should be exposed, but if we do it for element_rect(), we should do it for all elements.

[Yunuuuu]

Or we can add some documents, I found many user didn't know the alpha and fill_alpha function

[teunbrand]

Yeah let's go with just documenting how to set alpha on a colour. Alpha is separate from the colour/fill in geoms because they can be mapped from the data and thus be independent of a colour, whereas this generally doesn't occur with theme elements. Moreover, it would create some burden on extension developers to follow the same pattern in e.g. ggtext::element_markdown(), marquee::element_marquee(), ggh4x::element_part_rect() etc.