Improvements re: completing function as reference vs call

I think the last rounds of improvement re: function completions were in #819 and #754. For now, the heuristic for deciding whether to treat as a function reference vs. call is based on whether we're inside a list of special functions:

https://github.com/posit-dev/ark/blob/1577d04d2bc61e1484f8e0a57b99fe558b39a57c/crates/ark/src/lsp/completions/function_context.rs#L158-L166

Two examples of sites where a user wants a function reference (but gets parentheses anyway) came up in https://github.com/posit-dev/positron/issues/9044#issuecomment-3734419418

library(tidyverse)

penguins |> relocate(where(is.character()))
map(1:3, mean())

The current list only contains functions from the base and utils packages. But tidyselect::where() is a pretty clearcut case. More complicated, but also compelling is purrr::map() (and base equivalents). Not sure which will come first: improving the ad hoc list or improving the whole approach, but opening this as a place to track concrete examples that folks mention.

Read crates/ark/src/lsp/completions/function_context.rs around lines 158-166, then compare the existing special-function heuristic with the tidyselect::where() and purrr::map() examples in the issue. Review the referenced issues for context and determine whether the work should extend the list or replace the approach; done criteria are not specified beyond improving function-reference versus call completion behavior.