Checks for register_theme_element(element_tree)

[teunbrand]

This PR aims to fix #6162.

Aside from throwing an error in de issue's case, I also included stricter checks on the structure of the element_tree argument.
There is the possibility this might break packages that define their own theme elements, but if so, they've been using it incorrectly.

Reprex from issue:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
register_theme_elements(
  foobar = element_rect(),
  element_tree = list(foobar = el_def("element_rect", inherit = "foobar"))
)
#> Error in `register_theme_elements()`:
#> ! Invalid parent in `element_tree`: "foobar".

^{Created on 2024-10-29 with reprex v2.1.1}

[thomasp85]

LGTM