inline add_margins()

[teunbrand]

Hello there,

We have been preparing a new release of ggplot2 and during a reverse dependency check, it became apparent that the prospective ggplot2 3.5.0 would break shadowtext.

This PR copies the contents of the removed add_margins() function. Briefly, the use of unexported functions is discouraged, as ggplot2 makes no that they continue to exist. For more information about this issue, please see https://www.tidyverse.org/blog/2022/09/playing-on-the-same-team-as-your-dependecy/. While add_margins() is the one that broke shadowtext this time, there are others that are equally brittle found here:

shadowtext/R/element_shadowtext.R

Lines 106 to 111 in b810ebc

	"%||%" <- getFromNamespace("%||%", "ggplot2")
	zeroGrob <- getFromNamespace("zeroGrob", "ggplot2")
	modify_list <- getFromNamespace("modify_list", "ggplot2")
	add_margins <- getFromNamespace("add_margins", "ggplot2")
	rotate_just <- getFromNamespace("rotate_just", "ggplot2")
	font_descent <- getFromNamespace("font_descent", "ggplot2")

To test the code changes with the release candidate, you can install it with the code below:

remotes::install_github("tidyverse/ggplot2", ref = remotes::github_pull("5592"))

The release of ggplot2 3.5.0 is scheduled for the 12th of February. The progress of the release can be tracked in tidyverse/ggplot2#5588. We hope that this PR might help shadowtext get out a fix if necessary.

[GuangchuangYu]

thanks.