terraform_fmt: include .tfbackend files

As per the official docs https://developer.hashicorp.com/terraform/language/backend#file:

> *.backendname.tfbackend (e.g. config.consul.tfbackend) is the recommended naming pattern. Terraform will not prevent you from using other names but following this convention will help your editor understand the content and likely provide better editing experience as a result.

.pre-commit-hooks.yaml

@@ -13,7 +13,7 @@
     Rewrites all Terraform configuration files to a canonical format.
   entry: hooks/terraform_fmt.sh
   language: script
-  files: (\.tf|\.tfvars)$
+  files: (\.tf|\.tfbackend|\.tfvars)$
   exclude: \.terraform/.*$
 
 - id: terraform_docs