fix(mixins-preview): AutoDeleteObjects mixin fails with cannot find file error (#36188)

### Issue # (if applicable)

N/A

### Reason for this change



Synth fails when using the `AutoDeleteObjects` mixin:

```ts
import * as cdk from 'aws-cdk-lib';
import { Construct } from 'constructs';
import { CfnBucket } from 'aws-cdk-lib/aws-s3';
import { AutoDeleteObjects } from '@aws-cdk/mixins-preview/aws_s3/mixins';
import '@aws-cdk/mixins-preview/with';
import { Mixins } from '@aws-cdk/mixins-preview/core';

export class CdkSampleStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const bucket = new CfnBucket(this, 'MyBucket');
    Mixins.of(bucket).apply(new AutoDeleteObjects());
  }
}
```

Error messages:

```
❯ npx cdk synth
/Users/goto/cdk-sample/node_modules/@aws-cdk/mixins-preview/lib/custom-resource-handlers/aws-s3/auto-delete-objects-provider.ts:15
    return existing ?? new AutoDeleteObjectsProvider(stack, id, props);
                       ^
ValidationError: cannot find /Users/goto/cdk-sample/node_modules/@aws-cdk/mixins-preview/lib/custom-resource-handlers/dist/aws-s3/auto-delete-objects-handler/index.js
    at path [CdkSampleStack/Custom::S3AutoDeleteObjectsCustomResourceProvider] in aws-cdk-lib.CustomResourceProviderBase

    at Function.getOrCreateProvider (/Users/goto/cdk-sample/node_modules/@aws-cdk/mixins-preview/lib/custom-resource-handlers/aws-s3/auto-delete-objects-provider.ts:15:24)
    at AutoDeleteObjects.applyTo (/Users/goto/cdk-sample/node_modules/@aws-cdk/mixins-preview/lib/services/aws-s3/bucket.ts:27:48)
    at MixinApplicator.apply (/Users/goto/cdk-sample/node_modules/@aws-cdk/mixins-preview/lib/core/applicator.ts:29:17)
    at new CdkSampleStack (/Users/goto/cdk-sample/lib/cdk-sample-stack.ts:14:53)
    at Object.<anonymous> (/Users/goto/cdk-sample/bin/cdk-sample.ts:9:1)
    at Module._compile (node:internal/modules/cjs/loader:1554:14)
    at Module.m._compile (/Users/goto/cdk-sample/node_modules/ts-node/src/index.ts:1618:23)
    at node:internal/modules/cjs/loader:1706:10
    at Object.require.extensions.<computed> [as .ts] (/Users/goto/cdk-sample/node_modules/ts-node/src/index.ts:1621:12)
    at Module.load (node:internal/modules/cjs/loader:1289:32)
```

[AutoDeleteObjectsProvider](https://github.com/aws/aws-cdk/blob/v2.229.0/packages/@aws-cdk/mixins-preview/lib/custom-resource-handlers/aws-s3/auto-delete-objects-provider.ts#L21) specifies:

```ts
  private constructor(scope: Construct, id: string, props?: CustomResourceProviderOptions) {
    super(scope, id, {
      ...props,
      codeDirectory: path.join(__dirname, '..', 'dist', 'aws-s3', 'auto-delete-objects-handler'),
```

And the [script](https://github.com/aws/aws-cdk/blob/v2.229.0/packages/@aws-cdk/mixins-preview/scripts/airlift-custom-resource-handlers.sh) copies the dist files.

But [.npmignore](https://github.com/aws/aws-cdk/blob/v2.229.0/packages/@aws-cdk/mixins-preview/.npmignore) ignores `dist`, so code for auto-delete-objects-provider cannot be provided to user's module.

```
# There should be a 'dist' folder, but it is not there...

❯ ls node_modules/@aws-cdk/mixins-preview/lib/custom-resource-handlers/
  aws-s3
```

### Description of changes



### Describe any new or updated permissions being added



Added `!lib/custom-resource-handlers/dist` to packages/@aws-cdk/mixins-preview/.npmignore.

### Description of how you validated changes



### 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: go-to-k

packages/@aws-cdk/mixins-preview/.npmignore

@@ -6,6 +6,7 @@ coverage
 *.tgz
 
 dist
+!lib/custom-resource-handlers/dist
 .LAST_PACKAGE
 .LAST_BUILD
 !*.js