Make .buildggsurvplot an exported function

[NikNakk]

survminer:::.buildggsurvplot is useful for obtaining a gtable from a ggsurvplot for use in ggsave or for downstream manipulation. At present, it's a non-exported function and so the interface could be changed without notice in future, and is presumably not recommended for use in external production code. It would be helpful to consider exporting it, presumably without the leading . in the name.

This would help with issues such as #544 and #152.

[gernophil]

How can I use this function as long, as it is not exported?
survminer:::.buildggsurvplot as well as getFromNamespace(".buildggsurvplot", "survminer") raise Error: object '.buildggsurvplot' not found.