build: use the built and bundled javascript for main

[zimeg]

Type of change

• Bug fix

Summary

This PR updates the main in package.json to point to the dist/app.js that's created after running the following command:

$ npm run build

Fixes an issue of finding the built app with the @slack/cli-hooks defaults but does not build the app as part of the start hook!

Notes

This PR matches the changes in slack-samples/bolt-ts-starter-template#133 - will mirror whatever happens here!

Requirements

• I’ve checked my submission against the Samples Checklist to ensure it complies with all standards
• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct