Conflict between `base::is.element()` and `ggplot2::is.element()`

[teunbrand]

In #6022, we introduced the is.element() function to test if some object is an element_rect(), element_text(), element_blank() etc.:

ggplot2/R/theme-elements.R

Lines 205 to 207 in ddd207e

	#' @export
	#' @rdname is_tests
	is.element <- function(x) inherits(x, "element")

However, there is also base::is.element(), which acts as %in%. Since it is unlikely that base will rename that function anytime soon, we might have to come up with an alternative name for testing if an object is any of the theme elements. Should we just use is.theme_element()?