Add .suffix (and .prefix?) arguments to slides

[brookslogan]

• Adding suffixes seems the most natural naming scheme for epi_slide_{sum,mean,opt}. See also Add an intuitive new_col_name arg to epi_slide and epi_slide_* fns #461.
• Adding prefixes was missed in .f with tibble output and non-null new_col_name results in packed tibble column #534.

[brookslogan]

These seem to be kind of distinct options requiring different implementations and perhaps different argument names. One is based on input column names, the other on column names of data.frame slide comp outputs (prefixing non-data.frame output name doesn't make sense as there is no base output name for it to work off of).