Implement thousands comma-separators by default

[matt-dray]

There has been a request to:

...implement comma separated thousands format in the workbook, using openxlsx::createStyle(numFmt = “COMMA”) as per the guidance.

(Also, this is a reminder to look over the guidance in detail to extract all these kinds of cell-level features.)

[matt-dray]

This is kind of complex. The styling will occur when the column is full of numerics, but if it's character due to to a suppressed value, then the commas won't be shown.

Probably means that the thousands-separator styling will have to be dynamically applied by identifying the rows that have numbers in (e.g. coerce them to numeric and see if a non-NA result emerges).

[matt-dray]

Note the options for {openxlsx} could also be set to allow for the comma separator, as mentioned in #69, but this still won't work with columns that contain a placeholder or other string content.

[matt-dray]

Consider applying scales::number() if it needs to be done manually in some way.