Use an indent of 4 spaces for composer.json

Problem/Motivation

We use 2 spaces as indentation for composer.json file, but composer itself uses 4 spaces. This leads to unnecessary huge changes in git after people modify the composer.json via composer (e.g. composer require drupal/example) or use the composer_manager init script.

Proposed resolution

Use an indent of 4 spaces for all compsoer.json files and update .editorconfig accordingly.

Remaining tasks

User interface changes

API changes

Data model changes