Add allow-plugins in the skeleton config

[stof]

Refs composer/composer#10314

For now, the default value in Composer is still to allow all plugins implicitly for BC. But the behavior will change in July 2022 to default to an empty list of allowed plugins.

This PR has 2 effects:

• it makes new project use the new Composer behavior directly instead of relying on the BC layer
• it marks symfony/flex as an allowed plugin, as our skeleton is meant to be used with Flex

The first point means that projects installing other composer plugins will need to mark them as allowed for them to run (but composer update running in an interactive shell will ask for that anyway)

[nicolas-grekas]

We should also list symfony/runtime
This should target 4.4 don't you think?
As an additional step, we might want flex to generate these entries in existing projects.
Up for a PR doing so?

[stof]

I'll change the target branch to be 4.4
And yeah, symfony/runtime should indeed also be whitelisted in 5.3+

As an additional step, we might want flex to generate these entries in existing projects.

not needed. Composer will ask interactively about that when running Composer 2.2+ in an existing project AFAICT.

[stof]

I'll send another PR for symfony/runtime once this PR is merged and merged up inside the 5.3 branch (or 5.4 if we don't update 5.3 anymore).

[nicolas-grekas]

Thanks @stof
I'm going to add symfony/runtime while merging up, no worries.

[nicolas-grekas]

Thank you @stof.