Add .gitattributes excluding starlark from github's linguist (#31)

Not a priority by any means, but I think it would be nice if we can get
github to not put starlark in the list of languages used:

![image](https://github.com/user-attachments/assets/ebb8f991-2849-43c7-ba9f-8b790438c1b4)

I haven't actually tested this, but it's based on this documentation:
https://github.com/github-linguist/linguist/blob/main/docs/overrides.md#detectable

Author: ecioppettini

.gitattributes

@@ -0,0 +1 @@
+*.star -linguist-detectable