Fix the issues reported by phpcs

+/**
+ * The IntervalItemInterface interface.
+ */
 interface IntervalItemInterface {
 

That description just repeats the interface name without saying what the interface does.

   /**
+   * Set the default field storage backend for fields created during tests.
+   *
    * @inheritDoc
    */

Since that comment is edited, @inheritDoc must be replaced by {@inheritdoc}.

Issues should be assigned to somebody who creates a patch or a MR in the next hours, not after more than four hours. This means blocking an issue for which somebody else could have worked on.

Addressed #6. Please review.

    * @param \Traversable $namespaces
    *   An object that implements \Traversable which contains the root paths
-   *   keyed by the corresponding namespace to look for plugin implementations,
+   *   keyed by the corresponding namespace to look for plugin implementations,.

In English, a comma is not used before a period.

   /**
    * Profile to use.
+   *
+   * @var string
    */
   protected $profile = 'testing';
 
   /**
    * Theme to use.
+   *
+   * @var string
    */
   protected $defaultTheme = 'stark';

Since those comments are edited, the definite article that is missing in each description should be added too.

    * @var \Drupal\field\Entity\FieldStorageConfig
+   *   The FieldStorageConfig.
    */
   protected $fieldStorage;

The description must not repeat the class/interface name.

   /**
-   * @inheritDoc
+   * Set the default field storage backend for fields created during tests.

The first verb must use the third person singular.

-    // Verify initial field value
+    // Verify initial field value.

There is a missing definite article.

Fixed the issue based on #13 comment. Please review.

Addressed the comments partially on MR.

Issue summary updated.

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