force `*.re` and `*.rei` to resolve as Reason code

[Schniz]

GitHub currently references the test files as C++:

A small change in .gitattributes can let GitHub know that everything here is entirely Reason 😄

[fa7ad]

Hi! I realize this is a really small change, but I'd like to help nonetheless. Could you please assign the issue to me?

[Schniz]

No problems! Thanks for asking.

[fa7ad]

PR executed at #49.

I really appreciate adding the hacktoberfest label to the issue, if its not too much trouble could you add it to the repo as well? Looks like they changed the rules this time around to prevent spam entries.