fix: resolve @/ path aliases in tsup build config

The tsup build fails because esbuild cannot resolve the @/ path aliases
defined in tsconfig.json. Add esbuild alias configuration to tsup.config.ts
to map @ to the project root, matching the tsconfig paths setting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rafalzawadzki

tsup.config.ts

@@ -1,28 +1,36 @@
 import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
 import { defineConfig } from 'tsup';
 
 const package_ = JSON.parse(
   readFileSync(new URL('package.json', import.meta.url), 'utf8'),
 );
 
+const sharedConfig = {
+  define: {
+    __PACKAGE_VERSION__: JSON.stringify(package_.version),
+  },
+  esbuildOptions(options: { alias?: Record<string, string> }) {
+    options.alias = {
+      '@': resolve(import.meta.dirname),
+    };
+  },
+};
+
 export default defineConfig([
   {
     entry: ['src/index.ts'],
     format: ['cjs', 'esm'],
     dts: true,
     sourcemap: true,
-    define: {
-      __PACKAGE_VERSION__: JSON.stringify(package_.version),
-    },
+    ...sharedConfig,
   },
   {
     entry: ['src/internal/index.ts'],
     outDir: 'dist/internal',
     format: ['cjs', 'esm'],
     dts: true,
     sourcemap: true,
-    define: {
-      __PACKAGE_VERSION__: JSON.stringify(package_.version),
-    },
+    ...sharedConfig,
   },
 ]);