Add TermTypes class with term type constants

Useful when passing term types to services like TermBuffer.

Author: jakobw

RELEASE-NOTES.md

@@ -3,6 +3,7 @@
 ## Version 9.3.0 (development)
 
 * Raised minimum PHP version to 7.1
+* Added `TermTypes` with term type constants
 
 ## Version 9.2.0 (2020-01-24)
 

src/Term/TermTypes.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Wikibase\DataModel\Term;
+
+/**
+ * Term types used on Items and Properties
+ *
+ * @since 9.3
+ *
+ * @license GPL-2.0-or-later
+ */
+final class TermTypes {
+
+	public const TYPE_LABEL = 'label';
+	public const TYPE_DESCRIPTION = 'description';
+	public const TYPE_ALIAS = 'alias';
+
+}