Add AssetMapper and StimulusBundle for 6.4/7.0

[tacman]

I love AssetMapper. It is the front-end component we've been waiting for. Yes, webpack-encore was a good stepping stone, but required too much setup. If asset-mapper added here, more likely people will adopt it once they see how amazingly easy it is to include javascript libraries and CSS.

I also love stimulus, and would propose that symfony/stimulus-bundle is included in the pack as well. While it's not essential, it is always the second thing I install in a new project (after AssetMapper).

Of course, these make more sense for Symfony 6.4 and 7, since AssetMapper is marked as experimental before that.