[wb_load] minimal workbook

[JanMarvin]

This PR provides the safe part from #780

• is_wbWorkbook() and
• allows to set sheet = NULL in wb_load()

This is helpful to load a workbook without any sheet information. Think of it like this, you want only the sheet names in a hundred MB size xlsx file. Obviously you should be able to load all the sheets, but you shouldn't have to. The worksheet information is in workbook.xml so you do not need anything else. With this PR you can do the following:

# load only the bar minimum from a workbook
wb_load(x, sheet = NULL, data_only = TRUE)

Ideally this should be used in a few functions to restore a few openxlsx workflows like this - as suggested in our README

Furthermore, there are several ways to read certain information of an openxml spreadsheet without having opened it in a spreadsheet software before, e.g. to get the contained sheet names or tables.

Both of these should work interchangeable

library(openxlsx2)
example_file <- system.file("extdata", "openxlsx2_example.xlsx", package = "openxlsx2")
wb_load(example_file, sheet = NULL, data_only = TRUE)$get_sheet_names()
#>   Sheet1   Sheet2 
#> "Sheet1" "Sheet2"
wb_get_sheet_names(example_file)
#> Error: wb must be class wbWorkbook or R6