Add an icon for Stan

[WardBrian]

It would be great to support stan

There are icons available here: https://github.com/stan-dev/logos

[orta]

Doesn't look popular enough yet to really warrant being added:

[WardBrian]

That's fair; I think it's most common to use Stan in the context of R (see RStudio's support) so I'm not sure how many users there are in the VSCode world