feat: add environment variables support on deno.path

[ablause]

Adds the possibility to use environment variables on the Deno path in settings.

Resolves #377

[kitsonk]

Thanks for this, but I think the challenge is that it sort of provides some "bespoke" variable resolution. I would really prefer to be able to leverage something directly from the vscode platform versus trying to build something ourselves. There is this really old issue to add it to vscode which would be consistent with other parts of vscode, like the launch.json: microsoft/vscode#2809

[ry]

Closing because stale.