Add package.json to exports so it can be resolved

Many tools attempt to require.resolve package.json, and in the latest
versions of node that is not possible if the package specifies exports.
The solution is to simply add package.json to exports.

Resolves CesiumGS#8992

package.json

@@ -33,8 +33,11 @@
   "module": "./Source/Cesium.js",
   "types": "./Source/Cesium.d.ts",
   "exports": {
-    "require": "./index.cjs",
-    "import": "./Source/Cesium.js"
+    "./package.json": "./package.json",
+    ".": {
+      "require": "./index.cjs",
+      "import": "./Source/Cesium.js"
+    }
   },
   "type": "module",
   "devDependencies": {