IBX-2961: Refactored Extension to rely on Encore Configuration Dumper

[alongosz]

Question	Answer
JIRA issue	IBX-2961
Requires	ibexa/core#120
Type	improvement
Target Ibexa version	v4.1+
BC breaks	no

This PR aligns AdminUi with the changes done in ibexa/core#120 for ibexa/fieldtype-richtext#46

Doc

It is worth to update "Importing assets from a bundle" page, stating that now it's possible to place Ibexa Webpack Encore configuration in the project top-level directory encore. Applies to:

• ibexa.config.js
• ibexa.config.manager.js

And also:

• ibexa.webpack.custom.config.js
• ibexa.config.setup.js

but for the latter two I don't see any mention in the doc.

QA

1. Sanity/regression check regarding custom configuration mentioned in the doc and for those 4 files.
2. Sanity/regression check of OE (e.g. by creating an Article)

Checklist:

• Provided PR description.
• Tested the solution manually.
• Provided automated test coverage (generic coverage in core).
• Checked that target branch is set correctly (main for features, the oldest supported for bugs).
• Ran PHP CS Fixer for new PHP code (use $ composer fix-cs).
• Asked for a review