Support changes to R 4.2.0 help system

[kevinushey]

From wch/r-source@1c74636:

  \item The HTML help system has several new features: LaTeX-like
  math can be typeset using either \href{https://katex.org/}{KaTeX}
  or \href{https://www.mathjax.org/}{MathJax}, usage and example
  code are highlighted using \href{https://prismjs.com}{Prism}, and
  for dynamic help the output of examples and demos can be shown
  within the browser if the \CRANpkg{knitr} package is
  installed. These features can be disabled by setting the
  environment variable \code{_R_HELP_ENABLE_ENHANCED_HTML_} to a
  false value.

The enhanced help system looks good with light RStudio themes:

But not so great with dark RStudio themes:

We'll likely need to tweak our Help pane CSS to support the new Help rendering.

[ronblum]

Verified using ?Normal that text is displayed correctly using several light and dark themes in

• RStudio Desktop 2022.02.2+481 (Prairie Trillium)
• RStudio Server 2022.02.2+481 (Prairie Trillium)
• RStudio Desktop 2022.06.0-daily+295 (Spotted Wakerobin)

Also verified that no impact in R 4.1.3.