lexl: Tokenise 'Excel' Formulas

# From page 26 of Microsoft's published xlsx extensions [Excel (.xlsx) extensions to the office openxml spreadsheetml file format p.24](https://msdn.microsoft.com/en-us/library/dd922181(v=office.12).aspx).

#' Names of all Excel functions
#'
#' A dataset containing the names of all functions available in Excel.  This is
#' useful for identifying user-defined functions in formulas tokenized by
#' [lexl::lex_xl()].
#'
#' Note that this includes future function names that are already reserved.
#'
#' @format A character vector of length 600.
#' @source Pages 26--27 of Microsoft's document "Excel (.xlsx) extensions to the
#' office openxml spreadsheetml file format p.24"
#' \url{https://msdn.microsoft.com/en-us/library/dd922181(v=office.12).aspx},
#' revision 8.0 2017-06-20.
"excel_functions"