Add a dfe-specific diagnostics function

[cjrace]

Is your feature request related to a problem? Please describe.
It's fairly common for our team to need to check through certain environment variables that are known to cause issues on DfE machines while helping teams to diagnose and fix issues.

Describe the solution you'd like
A diagnostics function that prints out common variables such as the following list to the console:

• HTTP_PROXY
• HTTPS_PROXY
• no_proxy
• GITHUB_PAT
• RENV_DOWNLOAD_METHOD

I'm sure there'll be more to add to this to. Including does RTools exist, can it be found, and what's the location of key files such as the .Renviron and .Rprofile.

We could also add a verbosity argument, which if = full or true or something, returns full lists such as:

• renv::diagnostics()
• Sys.getenv()
• options()

Describe alternatives you've considered
Combining this with a clear all / unset all function, might be worth it.

Additional context
In the documentation (and console output) we should include the warning that some of the data printed may be sensitive, e.g. GitHub PATs, and not to share them more widely.