deparse rsconnect and local boards

[machow]

Part of #62 -- these are the simplest to do, and will make developing vetiver dockerfiles easier.

R implementations pasted below.

board_rsconnect("envvar", server = !!board$url)

board_deparse.pins_board_folder <- function(board, ...) {
  path <- check_board_deparse(board, "path")
  expr(board_folder(path = !!as.character(path)))
}

Let's implement a version that works for board_rsconnect as it exists right now, and handle the "envvar" auth stuff down the road after #93