Include help topics in completions for `?`

[jennybc]

I'm splitting this out of #1818, because it's a separate piece of work.

What we're talking about here is that in RStudio, if you attach dplyr, then trigger completions via ?, those completions include help topics (i.e. topics that aren't also exported functions).

Positron does not offer these perfectly valid completions.

Here's RStudio helping me read the help for dplyr-package and dplyr-locale:

Screen.Recording.2025-01-29.at.1.34.07.PM.mov

Positron gives the impression that it doesn't even know about these help topics with its completions (you also see double entries here 😬 which is tracked in #5708):

positron-help-topic-completion-trouble.mov

Just so I don't trap everything in videos, the examples above involve:

library(dplyr)
?dpl<COMPLETIONS OFFERED HERE>
# imagine you want to read the help for dplyr-package or dplyr-locale

We want the completions to help us get to topics like ?`dplyr-package`. This means that (1) dplyr-package and the like should appear in the list of completions and (2) Positron should also be smart enough to finesse the issue of backticks.