Enable default R location to be used on mac/linux if none is supplied

[gowerc]

What problem did you solve?

#339

(If you do not have screenshot) How can I check this pull request?

• Ensure R is on your search path
• open vscode with default settings and run "create new R terminal"
• a new R session should open
• change vscode default settings for R location on mac/linux to a null/junk location
• run "create new R terminal"
• A error message should appear saying unable to open the terminal

Please find a gif of the above below:

[gowerc]

Hey @andycraig & @renkun-ken, Updated the PR to look for the path variable across all 3 OS's. Works fine on my Mac but need someone to test it on windows as I don't have access to a windows machine sorry :(

[renkun-ken]

I test this on both macOS and Ubuntu and it works well. Maybe @andycraig could take a look at Windows.

[renkun-ken]

I test this on Windows 10 VM and it works as expected.

[Ikuyadeu]

@gowerc Great works! These have a lot of great improvements. I will publish a new version this night.