Look on the PATH to discover R installations on Windows

[jmcphers]

Followup from https://github.com/posit-dev/positron/discussions/3677.

Positron enumerates the registry to find R on Windows, which successfully discovers R installations that were installed in the traditional way (i.e. with the R installer). However, it's common on Windows to manually install R someplace else and put it on the $PATH.

This corresponds to this TODO:

https://github.com/posit-dev/positron/blob/c929d52f6635bd35e6fa773e3b397fde32f53097/extensions/positron-r/src/provider.ts#L340-L341