fix: imports from ESM modules cannot find correct type definitions (#23870)

Author: mrgrain

packages/aws-cdk-lib/package.json

@@ -407,6 +407,7 @@
   },
   "exports": {
     ".": {
+      "types": "./index.d.ts",
       "import": "./index.js",
       "require": "./lazy-index.js"
     },

tools/@aws-cdk/ubergen/bin/ubergen.ts

@@ -48,6 +48,7 @@ interface LibraryReference {
 }
 
 type Export = string | {
+  readonly types?: string;
   readonly import?: string;
   readonly require?: string;
 };
@@ -275,6 +276,7 @@ async function prepareSourceFiles(libraries: readonly LibraryReference[], packag
   // allowed for this package: we only want to allow the exact import statements that we want to support.
   packageJson.exports = {
     '.': {
+      types: './index.d.ts',
       import: './index.js',
       require: './lazy-index.js',
     },