Add .terragrunt-cache to skipDirNames (hashicorp#666)

In a large terragrunt live repo with many distinct configurations having
many cross dependencies, there can exist a large number of
`.terragrunt-cache` directories, each of which can contain a large
number of modules. There seems to me no good reason for language server
to walk these directories. In some case I was experiencing several
seconds of `formatOnSave` latency. This change removes that issue for
me.

internal/terraform/module/walker.go

@@ -26,6 +26,7 @@ var (
 		".idea":               true,
 		".vscode":             true,
 		"terraform.tfstate.d": true,
+		".terragrunt-cache":   true,
 	}
 )