Fix the error definition

Author: adjerbetian

lib/main.d.ts

@@ -90,10 +90,19 @@ export interface DotenvConfigOptions {
 }
 
 export interface DotenvConfigOutput {
-  error?: Error;
+  error?: DotenvError;
   parsed?: DotenvParseOutput;
 }
 
+type DotenvError = Error & {
+  code: 
+    | 'MISSING_DATA'
+    | 'INVALID_DOTENV_KEY'
+    | 'NOT_FOUND_DOTENV_ENVIRONMENT'
+    | 'DECRYPTION_FAILED'
+    | 'OBJECT_REQUIRED';
+}
+
 export interface DotenvPopulateOptions {
   /**
    * Default: `false`