Version bump, Style guide, dns records updates optional, and input validation

variable.domain_name.tf

@@ -1,4 +1,10 @@
 variable "domain_name" {
-  default = "example.com"
-  type    = string
+  default     = "example.com"
+  description = "The domain name for the Linode instance and domain records."
+  type        = string
+
+  validation {
+    condition     = can(regex("^[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$", var.domain_name))
+    error_message = "The domain name must be a valid fully-qualified domain name (e.g., example.com)."
+  }
 }