chore: move index.js to lib/test.js to match Node.js codebase (#20)

Author: aduh95

index.d.ts lib/test.d.tsindex.d.ts renamed to lib/test.d.ts

@@ -4,16 +4,16 @@
   "description": "Node.js 18's node:test, as an npm package",
   "license": "MIT",
   "repository": "nodejs/node-core-test",
-  "main": "./index.js",
-  "types": "./index.d.ts",
+  "main": "./lib/test.js",
+  "types": "./lib/test.d.ts",
   "bin": {
     "node--test": "./bin/node--test.js",
     "node--test-only": "./bin/node--test-only.js",
     "test": "./bin/node-core-test.js"
   },
   "imports": {
     "#internal/*": "./lib/internal/*.js",
-    "#node:test": "./index.js"
+    "#node:test": "./lib/test.js"
   },
   "scripts": {
     "test": "npm run test:lint && npm run test:unit",