fix: purrr::partial missing future arg placement

Use `... =` to specify this.  Prevents some confusing error messages on
```
archive_cases_dv_subset %>%
  epi_slide_opt(percent_cli, frollmean, 0, .window_size = 7)
```
but does not solve all issues that can happen with unnamed `...`.

Author: brookslogan

R/epi_slide_opt_archive.R

@@ -107,7 +107,7 @@ epi_slide_opt.epi_archive <-
         # debug stack traces:
         .f
       } else {
-        purrr::partial(.f, ...)
+        purrr::partial(.f, ... = , ...) # `... =` stands in for future args
       }
     col_names_quo <- enquo(.col_names)
     if (is.null(.window_size)) {

R/epi_slide_opt_edf.R

@@ -62,6 +62,7 @@ upstream_slide_f_info <- function(.f, ...) {
   }
   f_from_package <- f_info_row$package
   if (f_from_package == "data.table" && "fill" %in% names(rlang::call_match(dots_expand = FALSE)[["..."]])) {
+    # XXX this doesn't detect with `fill` is passed positionally through dots...
     cli_abort("`epi_slide_opt` does not support `data.table::froll*()` with a
                custom `fill =` arg",
       class = "epiprocess__epi_slide_opt__fill_unsupported"
@@ -600,7 +601,7 @@ epi_slide_opt.epi_df <- function(.x, .col_names, .f, ...,
       # debug stack traces:
       .f
     } else {
-      purrr::partial(.f, ...)
+      purrr::partial(.f, ... = , ...) # `... =` stands in for future args
     }
 
   result <- .x %>%