Create enum for FilterInterface:TYPE_* constants

Problem/Motivation

\Drupal\filter\Plugin\FilterInterface declares constants for each of the following filter types: TYPE_MARKUP_LANGUAGE, TYPE_HTML_RESTRICTOR, TYPE_TRANSFORM_REVERSIBLE, TYPE_TRANSFORM_IRREVERSIBLE. They happen to be integers because constants need a value, but since they are never serialized or written to a database we don't need them to be backed, except probably for BC.

Steps to reproduce

Proposed resolution

• Create a new enum FilterType with cases for MarkupLanguage, HtmlRestrictor, TransformReversible and TransformIrreversible.
• Update references to these constants to get the enum instead
• Add BC layer to find these by their former integer value
• Remove integer backing in a later version

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet