Groovy: Add .gvy, .grt, and .gtpl to the list of extensions

Recently, Groovy has received support for a template engine that is in pure Groovy Syntax. This PR just adds the general file extensions to the list that are used.
- .gtpl - Groovy Template (MarkupTemplateEngine Templates)
- .grt - Groovy Reactive Template (Allows Components)
- .gvy - Same as .groovy (A short alias to .groovy)