Add setnafill() support for character columns

Everything that was here is now merged; the TODO here was `setnafill()` support for `STRSXP`.

Closing this and marking for follow-up.

_Originally posted by @MichaelChirico in https://github.com/Rdatatable/data.table/issues/4980#issuecomment-3735079854_
            
`nafill()` supports all but raw, complex, but by-reference support for character is not yet implemented.