Add symfony/webpack-encore-bundle

[wbloszyk]

This PR will add webpack to sandbox, create some builds and add example build for sonata_admin (adminlte2).

Part of sonata-project/dev-kit#779

[wbloszyk]

@phansys this file should not be change here. I will open new PR in master branch for that.

[jordisala1991]

Not sure if this is what I thought of building the assets myself (same system as Sylius). On Sylius you dont have to create the js or css on your project, they are already on the downloaded composer files (vendor) you just have an main entry points and then you add them on your webpack config

[jordisala1991]

You can see it on: https://github.com/Sylius/Sylius-Standard

This sylius standard is something like our sandbox (everything installed and configured, ready to start a project)

[wbloszyk]

@jordisala1991 I know this it not what you mean. This will be done at final changes. For now concept is this branch is little diffrence:

People(contributors) can create own templates based on webpack build. For example I will do it for bootstrap4 and adminlte3.I will push templates to templates/bootstrap4 directory and webpack config to builds_configs/sonata_admin_adminlte3. Other contributors can check my work by run yarn encore dev on choosen config and by move templates to templates/bundle.

When some templates will be done, then we can split it to bundles and add webpack config as you wrote. At first we need working templates :)

[core23]

You can also add a new task to check if the assets can be build: https://github.com/nucleos/NucleosMapsBundle/blob/main/.github/workflows/continuous-integration.yml#L14-L31

[SonataCI]

Could you please rebase your PR and fix merge conflicts?