add_style_wrapper_billion: Generate a style with variable color or bolding
In ElliottMess/rapporteur: Reports on triple billion data

add_style_wrapper() adds a style for use within timeseries_style(), where the font color and decoration needs to vary based on data type. Wraps around openxlsx::addStyle() and openxlsx::createStyle().

1	add_style_wrapper_billion(wb, sheet_name, rows, cols, fgFill)

`wb`	a `openxlsx` workbook to be edited.
`sheet_name`	character name of the sheet to update
`rows`	Rows to apply style to, passed to `openxlsx::add_style()`.
`cols`	Columns to apply style to, passed to `openxlsx::add_style()`.
`fgFill`	Background color, passed to `openxlsx::createStyle()`.