[Rd] running examples with enhanced help engine also requires mime package

[Kevin Ushey]

Hi,

The new enhanced help system is fantastic! I've been playing around
with the enhanced help system introduced recently in R 4.2.0, and I
noticed one minor issue. When attempting to run examples from within a
Help page (in HTML help), if the knitr package is not installed, one
sees:

    To view output in the browser, the knitr package must be installed.

I think this is expected. However, after installing the 'knitr'
package and attempting to run examples again, I see:

    Error in loadNamespace(x) : there is no package called 'mime'

as it looks like 'knitr' (or a dependency?) is trying to use 'mime'
behind the scenes in the Help server. Because 'mime' is not an
explicit dependency of 'knitr', running install.packages("knitr")
isn't sufficient to ensure 'mime' is also installed (which a user
might have expected).

I'm not sure if this should be resolved in 'knitr' (e.g. via just
depending on the 'mime' package) or if this should be tweaked in R
itself.

Thanks,
Kevin