refactor: replace the Model->isset property with a public instance method (#9)

This change is primarily to accommodate the new `Reflect::serialize()` method since we don't want to include the isset property when returning serialized objects.

Reviewed-on: https://codeberg.org/vlw/scaffold/pulls/9

src/Database/Model.php

@@ -72,9 +72,8 @@
 		 * 
 		 * @return bool Entity exists
 		 */
-		public bool $isset {
-			// Returns bool if row is set or attempts to resolve and set if null
-			get => $this->_isset ??= new Database()
+		public function isset(): bool {
+			return $this->_isset ??= new Database()
 				->from($this->table)
 				->where($this->where)
 				->limit(1)