✨ Point and polygon theme elements

[teunbrand]

This PR aims to fix #6248.

It does the following:

• New element_point() and element_polygon() elements.
• New theme(point, polygon) arguments.
• Gave sensible defaults for these theme settings.

It does not do the following:

• Use these new elements in any meaningful way

[Yunuuuu]

I also require the element_polygon feature and have implemented it in my ggalign package to allows user to define links between two plots, as illustrated in the following figure (the blue and red polygons between the plots). I believe it would be more user-friendly if we could expose the alpha argument in element_polygon (and potentially in element_rect() as well).

[teunbrand]

Good to know I'm not the only one with this need :) I think exposing elements to alpha is a related but separate discussion that merits its own issue.