xl_workbook: A workbook: sheets plus workbook-level properties
Description
`xl_workbook()` binds one or more sheets (data frames or [xl_sheet]s) to a
set of [xl_properties]. It is the single place to attach workbook-level
formatting defaults and metadata. When passed to [write_xlsx()], the
workbook's `col_names` and `format_headers` settings take precedence over
`write_xlsx()`'s own arguments.