Circular register_theme_element() bug

[teunbrand]

Accidentally, I found myself in a situation where I defined a theme element with itself as its parent.
I couldn't reprex this bug because it halts execution of rmarkdown::render(), but here are the steps to reproduce:

library(ggplot2)
register_theme_elements(
  foobar = element_rect(),
  element_tree = list(foobar = el_def("element_rect", inherit = "foobar"))
)
calc_element("foobar", theme_get())
#> Error in match.fun(FUN) : node stack overflow
#> Error during wrapup: node stack overflow
#> Error: no more error handlers available (recursive errors?); invoking 'abort' restart

It is fine when I use inherit = "rect" or something that isn't the name of the element itself.
So it seems like defining an element with itself as its parent causes the bug.

This error was complicated to track down and the error message itself wasn't very helpful.
I propose we add a check to register_theme_elements() that throws an error when trying to define new elements that have themselves as parents.