fix: enhance accuracy of type definitions

[shawnsparks-work]

Issue #, if available:

Description of changes:
Updated type definitions of main package exports to more closely align with what is actually exported and allow common examples to work for consumers using TypeScript.

Examples that used to fail:

import { getCurrentInvoke } from '@vendia/serverless-express';

Simplified example from https://github.com/vendia/serverless-express/blob/mainline/examples/basic-starter-nestjs/src/lambda.ts

import serverlessExpress from '@vendia/serverless-express';
import express from 'express';

exports.handler = (event, context, callback) => {
    const handler = serverlessExpress( { app: express() } );
    return handler(event, context, callback);
};

Checklist

• Tests have been added and are passing
• Documentation has been updated

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

[shawnsparks-work]

I didn't add any tests because these are only changes to the type definitions and this package doesn't currently have typescript pulled in. I'm open to ideas on how to approach this.

[brett-vendia]

Thanks for this!

[github-actions]

🎉 This PR is included in version 4.3.4 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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