epic: Implement `cols_merge*()` family of methods

[machow]

cols_merge() and friends let you combine the content of two columns into one, with special templating syntax. E.g. if I have a mean column with value "X" and variance columns "Y.Z", then I might want to get X (Y.Z).

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

[machow]

• https://gt.rstudio.com/reference/fmt_url.html
• https://gt.rstudio.com/articles/case-study-clinical-tables.html
• pharma format sdtm: https://www.cdisc.org/standards/foundational/sdtm

Combining title, original name, and link

Note that when original name is NA, (<original_name>) is excluded.

Additional info in parentheses (e.g. stddev), ranges, etc..

More parentheses