Add type definition for third-party packages that do not have them.

source/packages.d.ts

@@ -0,0 +1,15 @@
+// Type definitions for third-party packages.
+
+declare module 'esbuild-copy-static-files' {
+  import {type cpSync} from 'node:fs';
+  import {type Plugin} from 'esbuild';
+
+  type CopySyncParameters = Parameters<typeof cpSync>;
+
+  type Options = {
+    src?: CopySyncParameters[0];
+    dest?: CopySyncParameters[1];
+  } & CopySyncParameters[2];
+
+  export default function (options: Options): Plugin;
+}