fix: export errors to make it available in region-info/lib/fact.ts (#34691)

### Issue #34690 

Closes #34690 

### Reason for this change
PR #33384 released in version `2.200.1` of `@aws-cdk/region-info` imported `UnscopedValidationError` from `aws-cdk-lib/core/lib/errors`, but did not export `errors.js` properly from `aws-cdk-lib`. This resulted in the following error message when attempting to `import { Fact } from '@aws-cdk/region-info'` (`fact.ts` imports `UnscopedValidationError`).

```
❯ node test.js
node:internal/modules/cjs/loader:1228
  throw err;
  ^

Error: Cannot find module 'aws-cdk-lib/core/lib/errors'
Require stack:
- /Volumes/workplace/test-region-info-app/node_modules/@aws-cdk/region-info/lib/fact.js
- /Volumes/workplace/test-region-info-app/node_modules/@aws-cdk/region-info/lib/index.js
- /Volumes/workplace/test-region-info-app/test.js
    at Module._resolveFilename (node:internal/modules/cjs/loader:1225:15)
    at Module._load (node:internal/modules/cjs/loader:1051:27)
    at Module.require (node:internal/modules/cjs/loader:1311:19)
    at require (node:internal/modules/helpers:179:18)
    at Object.<anonymous> (/Volumes/workplace/test-region-info-app/node_modules/@aws-cdk/region-info/lib/fact.js:8:18)
    at Module._compile (node:internal/modules/cjs/loader:1469:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1548:10)
    at Module.load (node:internal/modules/cjs/loader:1288:32)
    at Module._load (node:internal/modules/cjs/loader:1104:12)
    at Module.require (node:internal/modules/cjs/loader:1311:19) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/Volumes/workplace/test-region-info-app/node_modules/@aws-cdk/region-info/lib/fact.js',
    '/Volumes/workplace/test-region-info-app/node_modules/@aws-cdk/region-info/lib/index.js',
    '/Volumes/workplace/test-region-info-app/test.js'
  ]
}

Node.js v20.17.0
```

### Description of changes


Exported `errors.js` from `packages/aws-cdk-lib/package.json`, which makes it available to `@aws-cdk/region-info`.

### Describe any new or updated permissions being added
None.



### Description of how you validated changes



Tested by hand after making this code change, using example code from the linked issue.

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: iankhou

packages/aws-cdk-lib/package.json

@@ -506,6 +506,7 @@
     "./core/lib/helpers-internal": "./core/lib/helpers-internal/index.js",
     "./core/lib/metadata-resource": "./core/lib/metadata-resource.js",
     "./core/lib/prop-injectable": "./core/lib/prop-injectable.js",
+    "./core/lib/errors": "./core/lib/errors.js",
     "./custom-resources": "./custom-resources/index.js",
     "./custom-resources/lib/helpers-internal": "./custom-resources/lib/helpers-internal/index.js",
     "./cx-api": "./cx-api/index.js",