[WIP][DIC] Better handling of enableable configurations

See [the related comments](https://github.com/symfony/symfony/pull/6829#discussion_r2727742).

I think Symfony **must** get the configuration right as we can expect of lot of devs to use this as a template when writting their own configuration.

@schmittjoh could you please give me your feedback on [this change](https://github.com/vicb/symfony/pull/new/fmwk/config#L4R224) considering [the rationale](https://github.com/vicb/symfony/pull/new/fmwk/config#L3R7).

@fabpot this is a WIP as tests do not pass as the configurations are not processed. Could you please take a look at how this should be solved, I won't have time to work on this more before next week and I think this is important as explained in introduction.

Todo:

[ ] Complete the PR
[ ] Add the PR header