[Stimulus] No default asset manager lib

[Nayte91]

Hello there,

This is between the bug and the feature: I played with a new project, doing a create symfony skeleton, without "webapp".
Then I made a "composer req stimulus".
Then... It exploded, because stimulus doesn't require asset-mapper or webpack Encore as a dependency, but it needs it anyway to work.

So I wonder if we should check for either package A or B are here, and if not, require A? It's doable with composer provide, as we can define a virtual package like "symfony/asset-management" (arbitrary name) as an interface. Then, webpack Encore and Asset-mapper's composer.json would have this entry:

 "provide": {
        "symfony/assets-management": "^7.3"
    }

I'm not sure how to tell to require asset-mapper if neither asset-mapper nor webpack-encore are installed, but do we want it to happen at a first step?