epic: Implement `fmt_units()` and `cols_units()`

[machow]

These functions use a Great Tables specific mini language to specify units in labels. Note that functions like md() can do a lot of unit stuff already (like creating superscripts), but it's more limited (e.g. some combos of subscripts and superscripts) and this allows Great Tables to render to things like latex, etc..

https://gt.rstudio.com/reference/cols_units.html