feat(core): add codeOf utility and update exports and tests

Added `codeOf` utility to simplify retrieving error codes from `DefinedError`. Updated exports in `index.ts` to include all `of.ts` utilities and enhanced `neverthrow.test.ts` to validate error code retrieval.

Author: tarnishablec

packages/core/__tests__/neverthrow.test.ts

@@ -1,6 +1,6 @@
 // noinspection ES6UnusedImports
 import { describe, expect, test } from "bun:test";
-import { PayloadField, payloadOf, type ThatError } from "@thaterror/core";
+import { codeOf, PayloadField, payloadOf, type ThatError } from "@thaterror/core";
 import { err, fromPromise } from 'neverthrow'
 import { AppError } from './define-error.test'
 
@@ -40,6 +40,8 @@ describe('neverthrow test', () => {
             expect(result.error.is(AppError.NotFound)).toBe(true);
             const [id] = payloadOf(result.error);
             expect(id).toBe(404);
+            const code = codeOf(result.error);
+            expect(code).toBe("NotFound");
         }
     })
 })

packages/core/src/index.ts

@@ -1,5 +1,5 @@
 export { That } from './define'
 export { isDefinedError } from './is'
-export { payloadOf, scopeOf } from './of'
+export * from './of'
 
 export * from './types'

packages/core/src/of.ts

@@ -1,4 +1,4 @@
-import { type DefinedError, type ErrorCase, type ErrorFamily, PayloadField, ScopeField } from "./types";
+import { CodeField, type DefinedError, type ErrorCase, type ErrorFamily, PayloadField, ScopeField } from "./types";
 
 export interface Scoped {
     readonly [ScopeField]: symbol;
@@ -18,4 +18,8 @@ export function scopeOf(target: Scoped): symbol {
 
 export function payloadOf<E extends DefinedError>(error: E): E[typeof PayloadField] {
     return error[PayloadField];
+}
+
+export function codeOf<E extends DefinedError>(error: E): E[typeof CodeField] {
+    return error[CodeField];
 }