v2.0 `modify_footnote()` updates

[ddsjoberg]

• I think footnotes and abbreviations need to be handled with different functions. It's confusing having them together. It would be great if the abbreviations were not a regular footnote. Most tables define abbreviations in more of a source note way.
• I would like modify_footnote() to be able to place markers in the body of the table.

[ddsjoberg]

I wonder if it'd be best to create a new function modify_footer() and modify_abbrevation() that would

• Handle multiple footnotes on the same cell
• Put footnotes in headers, body of the table, and spanning headers
• Treat abbreviations more like source notes.

It would be better to keep modify_footnote() but change the API in a hopefully non-breaking way.

modify_footnote(
   x, 
  ..., 
  location = c("header", "body", "spanner", "title", "subtitle"),  # would need to specify one of these,
  rows = NULL, # only used when location='body'
  spanner_level = 1L, # only used when location='spanner'
  replace = TRUE,  # the current implementation only allows one footnote per cell, so it allows replaces the existing
  text_interpret = c("md", "html")
)

We can keep the same syntax for deleting footnotes, e.g. colname = NA

---

Maybe it would be easier to simply export footnote functions for each location:

modify_footnote_header()
modify_footnote_body()
modify_footnote_spanner()
modify_footnote_title() # this would depend on whether we go for titles or captions (i don't think we can add footnotes to captions?)
modify_footnote_subtitle()