fix: add package.json exports to avoid DEP0151 warning (#6168)

* fix: add package.json exports to avoid DEP0151 warning

Add an explicit exports map so Node stops falling back to
deprecated default index lookups for ESM package resolution.
Keep the wildcard subpath export to preserve existing deep imports
and browser bundle accessibility.

Fixes: #6147

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

* format

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: mark-wiemer

package.json

@@ -31,6 +31,10 @@
   "homepage": "https://mochajs.org/",
   "logo": "https://cldup.com/S9uQ-cOLYz.svg",
   "notifyLogo": "https://ibin.co/4QuRuGjXvl36.png",
+  "exports": {
+    ".": "./index.js",
+    "./*": "./*"
+  },
   "bin": {
     "mocha": "./bin/mocha.js"
   },

test/integration/package-entrypoint.spec.cjs

@@ -0,0 +1,50 @@
+"use strict";
+
+const fs = require("node:fs");
+const os = require("node:os");
+const path = require("node:path");
+const { invokeNode } = require("./helpers.cjs");
+
+describe("package entrypoint", function () {
+  it("should resolve the package root without DEP0151 warnings", async function () {
+    const repoRoot = path.resolve(__dirname, "..", "..");
+    const tempDir = fs.mkdtempSync(
+      path.join(os.tmpdir(), "mocha-package-root-"),
+    );
+    const packageDir = path.join(tempDir, "node_modules", "mocha");
+
+    fs.mkdirSync(path.dirname(packageDir), { recursive: true });
+
+    try {
+      if (process.platform === "win32") {
+        fs.symlinkSync(repoRoot, packageDir, "junction");
+      } else {
+        fs.symlinkSync(repoRoot, packageDir);
+      }
+
+      const result = await new Promise((resolve, reject) => {
+        invokeNode(
+          [
+            "--input-type=module",
+            "-e",
+            "import {describe, it} from 'mocha'; console.log(typeof describe, typeof it);",
+          ],
+          (err, res) => {
+            if (err) {
+              reject(err);
+              return;
+            }
+            resolve(res);
+          },
+          { cwd: tempDir, stdio: ["ignore", "pipe", "pipe"] },
+        );
+      });
+
+      expect(result.code, "to be", 0);
+      expect(result.output, "to contain", "function function");
+      expect(result.output, "not to contain", "DEP0151");
+    } finally {
+      fs.rmSync(tempDir, { recursive: true, force: true });
+    }
+  });
+});