Allow workspaces to open with "the current version of R" without any fuss

[jmcphers]

Currently, Positron saves a very exact version of R used with each workspace (as the "affiliated runtime"), and attempts to restore it when the workspace is started up.

However, in many (most?) situations, the desire isn't necessarily to use that very specific version of R. R users are accustomed to using RStudio, which just picks up whatever's current.

Proposal: Allow Positron workspaces to bind to R in such a way that they can use whatever the current version is without any fuss/warnings.

We could do this with changes only to the R language pack:

• When building R language metadata, we add a flag to the "additional fields" (alongside the home path and bin path) indicating whether the version of R is the current default on the machine.
• When Positron asks the R language pack to validate metadata, it checks to see if the R version being sought was the current version when the metadata was saved. If it is, the R language pack returns updated metadata corresponding to the new current R version, if it has changed.

Related to (and perhaps an enhancement of) #2316.

[jennybc]

I think we decided that #2652 addressed this.