Add RPCs for package documentation tools

[jmcphers]

This change ports some package documentation tools from the btw package into ark, exposing them as RPCs that we can call from Positron (where we'll hook them up to language model tool calls).

Specifically, the changes from btw are as follows:

• remove dependency on tidyverse (dplyr, etc.) by rewriting transformations in base R
• return plain-text results rather than throwing errors in most cases
• remove dependency on rmarkdown by bundling the needed pandoc conversion bits

Source for most RPCs: https://github.com/posit-dev/btw/blob/main/R/tool-docs.R

Part of posit-dev/positron#8016.

[lionel-]

I did not do a deep review as this is mostly ported code and I'm not super familiar with the help system. But it looks good and I've tested the various RPCs. Only thing to note is that an error message has an interpolation bug:

.ps.rpc.get_help_page("list5", "rlang")
#> [1] "No help page found for topic list5 in package {.pkg {package_name}}."

@jmcphers Do you think it would be helpful for the rmarkdown and btw package to implement the tools used here in standalone files? These look like this: https://github.com/r-lib/rlang/blob/main/R/standalone-purrr.R

Since the standalone file is in the original repo and has clear dependency goals (for example only base R) that the maintainer abides to, this makes it easy to update upstream changes from downstream. We even have tooling for updating these files that manages dependencies (standalone files may depend on other standalone files): https://usethis.r-lib.org/reference/use_standalone.html.

[jmcphers]

an error message has an interpolation bug:

Thanks, fixed in 27b3fbb!

Do you think it would be helpful for the rmarkdown and btw package to implement the tools used here in standalone files?

Yes, it would be helpful! I didn't know about this mechanism.

[lionel-]

@jmcphers I suggested implementing in standalone files in:

posit-dev/btw#78
rstudio/rmarkdown#2596