fix(backend): enable tree shaking in esbuild

[trivikr]

Issue

Follow-up to https://github.com/aws-samples/aws-sdk-js-notes-app/pull/40

Description

Enable tree shaking in ESBuild by preferring module field before main. Docs: https://esbuild.github.io/api/#main-fields

Before

```console $ yarn build:backend dist/updateNote/app.js 498.4kb dist/createNote/app.js 498.4kb dist/getNote/app.js 498.3kb dist/deleteNote/app.js 498.3kb dist/listNotes/app.js 498.2kb ⚡ Done in 102ms ```

After

```console $ yarn build:backend dist/updateNote/app.js 243.0kb dist/createNote/app.js 242.8kb dist/deleteNote/app.js 242.4kb dist/getNote/app.js 240.6kb dist/listNotes/app.js 238.5kb ⚡ Done in 107ms ```

Testing

Verified that notes application works.

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