chore(oxfmt-config): migrate build to tsdown

internal/lint-configs/oxfmt-config/tsdown.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from 'tsdown';
+
+export default defineConfig({
+  clean: true,
+  dts: true,
+  entry: ['src/index.ts'],
+  format: ['esm'],
+  outExtensions: () => ({
+    dts: '.d.ts',
+  }),
+});