Add CommentPreview enum and replace usage of DRUPAL_DISABLED, DRUPAL_OPTIONAL and DRUPAL_REQUIRED

Problem/Motivation

Part of #2851705: [meta] Deprecate DRUPAL_DISABLED, DRUPAL_OPTIONAL and DRUPAL_REQUIRED - we want to deprecate DRUPAL_DISABLED, DRUPAL_OPTIONAL and DRUPAL_REQUIRED. Doing this in a single issue is too hard to review and we want to change the API as we go.

This ticket is for the Comment module.

Proposed resolution

- Add CommentPreview enum
- Replace usages of constants with new enum
- Update $mode param in CommentTestBase::setCommentPreview to take CommentPreviewMode enum, throw a deprecation when its an integer.
- $message is unused by setCommentSettings so we can remove $mode_text entirely. We can used named params to continue to pass $field_name. See #3478418: Deprecate $message param from \Drupal\Tests\comment\Functional\CommentTestBase::setCommentSettings
- Pass $mode->value into setCommentSettings

API changes

CommentTestBase::setCommentPreview now takes a CommentPreviewMode enum instead of int for $mode

Release notes snippet

N/A