Fix PrintUtil for multi-valued attributes

[vhemery]

Displays multi-valued attributes correctly.

The example plugin does not have tests, and registered models do not have mutivalued attributes.
Adding a new test module may not be relevant.
However, one can simply test this fix by executing :

package org.eclipse.emfcloud.modelserver.example.util;

import java.util.List;

import org.eclipse.emf.common.util.EList;
import org.eclipse.emf.ecore.EcoreFactory;
import org.eclipse.emf.ecore.EcorePackage;
import org.eclipse.emf.ecore.util.EcoreUtil;

public class PrintUtilTest {

   public static void main(final String[] args) {
      var ep = EcoreFactory.eINSTANCE.createEPackage();
      ep.setName("testModel");
      var ec = EcoreFactory.eINSTANCE.createEClass();
      ec.setName("TestManyAttribute");
      var att = EcoreFactory.eINSTANCE.createEAttribute();
      att.setName("values");
      att.setEType(EcorePackage.Literals.ESTRING);
      att.setUpperBound(-1);
      ec.getEStructuralFeatures().add(att);
      ep.getEClassifiers().add(ec);

      var obj = EcoreUtil.create(ec);
      EList<String> values = (EList<String>) obj.eGet(att);
      values.addAll(List.of("1", "2", "3"));
      // displays value successfully as "values: [1, 2, 3]"
      System.out.println(PrintUtil.toPrettyString(obj));
   }

}

Fixes #276

[vhemery]

Needs rebase after #280