Feature request: export "main" entrypoint to allow using it directly instead of using the cli

I'm interested in using the main entrypoint function directly instead of using the cli to start up the function, mostly on the development environment.

This has some advantages, among others:

- Can be started with node directly: `node src/server.js`
- Can use node watcher, no need for extra setup with nodemon&others: `node --watch src/server.js`
- Better typescript support with ts-node/tsx: `tsx --watch src/server.ts`

This server.js would use the exported main entry point:

```js
import {main} from 'functions-framework'

import {handle} from './index.js'

main(handle)
```

I'd be willing to implement this feature