Support ESM module export for @azure/functions-extensions-servicebus

## Summary

The package `@azure/functions-extensions-servicebus` does not export an ESM module, which forces consumers using ESM (`"type": "module"` in `package.json`) to set `verbatimModuleSyntax: false` in their `tsconfig.json`. This is not ideal for modern TypeScript projects.

## Current Behavior

- `package.json` exports both `import` and `require` pointing to the same CommonJS file:
```json
"exports": {
".": {
"import": "./dist/azure-functions-extensions-servicebus.js",
"require": "./dist/azure-functions-extensions-servicebus.js"
}
}
```
- The bundle is built with `libraryTarget: 'commonjs2'` (webpack)
- No `"type": "module"` in `package.json`
- No separate ESM output exists

## Expected Behavior

The package should provide a proper ESM export (e.g., separate `.mjs` or dual-package build) so that consumers can use it with `verbatimModuleSyntax: true` and ESM-native setups without workarounds.

## Customer Report

> The package does not export an ESM module, which forces us to set `verbatimModuleSyntax` to `false`. This isn't ideal for our setup. Are there any plans to support ESM modules properly?

## Workaround

Set `verbatimModuleSyntax: false` in `tsconfig.json`.

## Related Issues

- Azure/azure-functions-nodejs-worker#104 — Add ES6 Style Exports (OPEN, P1)
- Azure/azure-functions-nodejs-library#287 — Provide package as ESM alongside CommonJS module (OPEN, P1)
- Azure/azure-functions-nodejs-library#396 — Type error when using `verbatimModuleSyntax: true` (CLOSED)
- Azure/azure-functions-nodejs-library#397 — `.d.ts` files should not import `.ts` files (related)
- #26 — esbuild bundler compatibility issue (related but different)

## Context

The Azure Functions Node.js worker itself is CJS-based, and `@azure/functions-extensions-base` is also CJS-only. Full ESM support for this extension likely depends on the upstream worker and library ecosystem supporting ESM first.