Support downlit-style auto-linking for R function and package references

[cwickham]

The downlit R package auto-links package and function references in rendered R content. Two flavors:

1. Code-block autolinks — function names inside R code blocks become hyperlinks to their docs (rdrr.io for base R / CRAN packages, pkgdown sites for tidyverse-style packages).

   Example:

   <a href="https://rdrr.io/r/utils/install.packages.html">install.packages</a>("bslib")

2. Prose autolinks — bare-word mentions of "attached" packages in prose link to the package homepage. Triggered by an options(download.attached = c("shiny", "bslib")) chunk in the source .qmd.

   Example: every plain-text bslib mention becomes <a href="https://rstudio.github.io/bslib/">bslib</a>.

The current Hugo site renders neither flavor. Supporting them would benefit ported posts (authored expecting downlit) and any new R-content posts.

Ported posts where downlit-generated links are missing

Post	Local	Legacy
bslib-dashboards	opensource.posit.co	shiny.posit.co
bslib-tooltips	opensource.posit.co	shiny.posit.co
shiny-r-1.8.0	opensource.posit.co	shiny.posit.co
shiny-r-1.8.1	opensource.posit.co	shiny.posit.co