[BUG] add_fct could generate unparsable file

[VincentGuyader]

In golem, when a user runs golem::add_fct(name = "ma fonction") with a space in the name, it generates both a file with a space in its name and a function placeholder that includes the space. This results in invalid code.

For example:

#' ma fonction 
#'
#' @description A fct function
#'
#' @return The return value, if any, from executing the function.
#'
#' @noRd
ma fonction <- function() {
}

We'll take this opportunity to enforce lowercase conversion and transliteration in the file name.