Add `.prql` to linguist

[eitsupi]

Of course this does not need to be done right now (and does not meet the submission criteria).

Registering syntax in linguist will activate syntax highlighting on GitHub.
https://github.com/github/linguist/blob/master/CONTRIBUTING.md#adding-an-extension-to-a-language
https://github.com/github/linguist/blob/master/lib/linguist/languages.yml

[eitsupi]

This is the search result of the prql extension on GitHub.
https://github.com/search?q=path%3A*.prql&type=code

There are 296 files now (linguist requires 2000)

[ofek]

Now it's 400.