[feat] format numbers and dates (range_format_pattern)

[mattle24]

Proposed as fixes #81, partially fixes #179 .

New range_format_pattern() to be able to format date and number data in Sheets.

@jennybc we talked about this a few months ago in #81, please let me know what you think of this, even if it's just this is not the path forward for googlesheets4! I did try an approach to formatting using vctrs (mattle24#1), but I found it to be less flexible for supporting both numbers and dates, since the types are separate in R but similar in Google Sheets. Also, I think that formatting directly on the Google Sheet, rather than manipulating the R data frame, could make it easier to add more formatting (color, alignment, etc.) down the road.

Thanks for the work on the package, it's really useful!

[jennybc]

Thanks for exploring this @mattle24. As we discussed elsewhere, I doubt this will get merged.

But a concrete PR definitely helps everyone pre-think about what this feature should look like, it shows exactly what you have in mind, etc. So it will accelerate progress on getting this sort of functionality, which is in the plan.

[mattle24]

Sounds great, thanks @jennybc !