(aws-lambda-nodejs): `.mjs` file extension and `import.meta` not supported

Although Lambda now supports Node v14:

1. .mjs entry files are rejected by a regex pattern in aws-lambda-nodejs
2. esbuild is transpiling the scripts to cjs format (commonjs) but BundlingOptions provides no means to specify format: "esm", and consequently esbuild polyfills import.meta which breaks all of its uses in the scripts

Reproduction Steps

lib/my-stack.mjs:

export class MyStack extends Stack {
    constructor(scope, id, props) {
        super(scope, id, props);

        // ...

        new NodejsFunction(this, 'Example', {
            runtime: Runtime.NODEJS_14_X,
            entry: 'src/entry-file.mjs',
            bundling: {
                target: 'es2020',
                // format: 'esm',   <-- should be able to pass this option here
            },
        };

        // ...
    }
}

src/entry-file.mjs:

export async function Example() {
    return import.meta.url;
}

What did you expect to happen?

1. entry-file.mjs to be allowed to be used as an entry file.
2. import.meta to be defined and Example() to return a string.

What actually happened?

1. Error: Only JavaScript or TypeScript entry files are supported.
2. Example() returns undefined since import.meta is polyfilled with an empty plain object.

Environment

• CDK CLI Version : n/a
• Framework Version:
• Node.js Version: 14.13.0
• OS : n/a
• Language (Version):

Other

1. For the .mjs extension: https://github.com/aws/aws-cdk/blob/5d71d8e815529ccde7ffe13bb72f57ae88e896dc/packages/%40aws-cdk/aws-lambda-nodejs/lib/function.ts#L140

2. Allow passing in the format option to esbuild: https://github.com/aws/aws-cdk/blob/5d71d8e815529ccde7ffe13bb72f57ae88e896dc/packages/%40aws-cdk/aws-lambda-nodejs/lib/bundling.ts#L141-L158

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This is 🐛 Bug Report

[ronaldocpontes]

This is what I get when using “type”: “module” on package.json. As you can see on the error: require() of ES modules is not supported. require() of /var/task/index.js…

The lambda engine is using require() to load the code on index.js.

# package.json
{
"name": "myLambda",
"type": "module",
"main": "index.js",
}

# index.js
export const handler = async (event) => {  
    const response = {
        statusCode: 200,
        body: JSON.stringify('Hello from Lambda!'),
    };
    return response;
};

Response
{
  "errorType": "Error",
  "errorMessage": "Must use import to load ES Module: /var/task/index.js\nrequire() of ES modules is not supported.\nrequire() of /var/task/index.js from /var/runtime/UserFunction.js is an ES module file as it is a .js file whose nearest parent package.json contains \"type\": \"module\" which defines all .js files in that package scope as ES modules.\nInstead rename index.js to end in .cjs, change the requiring code to use import(), or remove \"type\": \"module\" from /var/task/package.json.\n",
  "trace": [
    "Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: /var/task/index.js",
    "require() of ES modules is not supported.",
    "require() of /var/task/index.js from /var/runtime/UserFunction.js is an ES module file as it is a .js file whose nearest parent package.json contains \"type\": \"module\" which defines all .js files in that package scope as ES modules.",
    "Instead rename index.js to end in .cjs, change the requiring code to use import(), or remove \"type\": \"module\" from /var/task/package.json.",
    "",
    "    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1080:13)",
    "    at Module.load (internal/modules/cjs/loader.js:928:32)",
    "    at Function.Module._load (internal/modules/cjs/loader.js:769:14)",
    "    at Module.require (internal/modules/cjs/loader.js:952:19)",
    "    at require (internal/modules/cjs/helpers.js:88:18)",
    "    at _tryRequire (/var/runtime/UserFunction.js:75:12)",
    "    at _loadUserApp (/var/runtime/UserFunction.js:95:12)",
    "    at Object.module.exports.load (/var/runtime/UserFunction.js:140:17)",
    "    at Object.<anonymous> (/var/runtime/index.js:43:30)",
    "    at Module._compile (internal/modules/cjs/loader.js:1063:30)"
  ]
}

[jogold]

https://aws.amazon.com/about-aws/whats-new/2022/01/aws-lambda-es-modules-top-level-await-node-js-14/

https://aws.amazon.com/blogs/compute/using-node-js-es-modules-and-top-level-await-in-aws-lambda/