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

Start by tracing the existing CLI startup path and the exported main entry point in functions-framework. The request proposes src/server.js or src/server.ts as a direct Node entrypoint; done means users can invoke main directly while preserving the current CLI behavior.

Written by the indexing model from the issue text.