feat: add DLU_CONFIG_DIR env var (#1370)

* feat: add DLU_CONFIG_DIR env var

* fix: PascalCase

dCommon/dConfig.h

@@ -7,6 +7,11 @@ class dConfig {
 public:
 	dConfig(const std::string& filepath);
 
+	/**
+	 * Checks whether the specified filepath exists
+	 */
+	static const bool Exists(const std::string& filepath);
+
 	/**
 	 * Gets the specified key from the config.  Returns an empty string if the value is not found.
 	 *