User configurable R location

[jennybc]

Addresses #2235

Adds 2 configuration points:

• positron.r.customRootFolders is an array. Each entry is a folder that is searched for 1 or more R installations.
• positron.r.customBinaries is an array. Each entry is the path to a specific R binary.

QA Notes

It's hard to get into a position where this configuration comes into play, because you have to make sure that four methods of automatic discovery won't work. I'm focusing on Windows because that's the main OS that drives the need for this.

• Install R at a non-standard location. This will have to be via the CRAN installer since rig doesn't give you any choice about this. Therefore I removed my rig-installed released version of R and re-installed released R with the CRAN installer below C:/nonstandardRLocation/. Make sure to uncheck the box to write to the registry! If you forget or you're doing this with an existing installation, you can remove the registry keys with C:/nonstandardRLocation/R-4.4.1/bin/x64/RSetReg.exe /U (note the /U argument for unsetting keys). All R installations on Windows ship with this executable for setting/unsetting registry keys.
• Now use rig (or other means) to make some other R version the current version. In my setup, R 4.2.3 and 4.3.3 are good candidates for the new current version of R, e.g. rig default 4.3.3.
• Make sure this R installation is not on the PATH.
• Fire up Positron. The interpreter drop down should not include this R version that is
  • not in the standard root folder for R, i.e. not in C:/Program Files/R nor in C:/Users/<Username>/AppData/Local/Programs/R
  • not the current version of R
  • not recorded in the registry
  • not on the PATH

Use the command palette to do Preferences: Open Settings. Go to Extensions > R > Positron R Language Pack (advanced) and do each of these, in turn:

• In Custom Binaries, add the path to the nonstandard R binary, e.g. C:/nonstandardRLocation/R-4.4.1/bin/x64/R.exe
• In Custom Root Folders, add the parent folder of the nonstandard R version, e.g. C:/nonstandardRLocation

Fire up Positron again. Either of these settings should make the nonstandard installation appear in the interpreter dropdown.

Other things that can be noticed in the Positron R output channel:

• In the presence of custom binaries, you'll see something like this:

  2024-10-30 01:03:07.761 [info] User-specified R binaries:
  [
    "C:/nonstandardRLocation/R-4.4.1-bin/x64.R.exe"
  ]
  

• In the presence of a custom root folder, you'll see something like this:

  2024-10-30 00:56:15.137 [info] User-specified directories to scan for R installations:
  [
    "C:/nonstandardRLocation"
  ]
  

[jennybc]

Just continuing to lay groundwork that will be helpful when tackling #1397.

[jennybc]

This is an unrelated improvement. Part of continuing effort to reduce rediscovery and repetition in the output channel.

[juliasilge]

From a code perspective and how the options are presented to users, this is looking great!

I did just spend a bit of time trying to install R in a non-standard location on my dev machine AND FAILED 😆 so let's get this merged in and available for testing in a broader set of circumstances.

[jennybc]

Heads up @kevinushey: as previously discussed, you can now configure this location via the new positron.r.customRootFolders configuration point, available in the advanced section of positron-r's settings.