[AssetMapper] Allow to define entrypoint in importmap.php

[nicolas-grekas]

Let's document symfony/flex#1026

/cc @Jibbarth @symfony/ux

[javiereguiluz]

In order to document this, please show a real example of how to use this.

For example, consider an ExampleBundle that defines assets/styles/foo.css and assets/bar.js files.

What we need to know:

• What exact config and in which file is needed for this bundle to expose both files in Symfony apps that install this bundle?
• Which exact statement must this bundle use in its own templates to load these assets using importmap()?
• Which exact statement must Symfony applications using this bundle add in the templates defined in templates/ to load these assets using importmap()?

Thanks!