PHP 8.5 | Image constants: add IMAGETYPE_HEIF constant

Document the `IMAGETYPE_HEIF` constant in the image extension constants reference.

Refs:
* php/php-src#18940
* php/php-src#20713

Author: rodrigoprimo

reference/image/constants.xml

@@ -701,6 +701,18 @@
     </simpara>
    </listitem>
   </varlistentry>
+  <varlistentry xml:id="constant.imagetype-heif">
+   <term>
+    <constant>IMAGETYPE_HEIF</constant>
+    (<type>int</type>)
+   </term>
+   <listitem>
+    &gd.constants.type;
+    <simpara>
+     (Available as of PHP 8.5.0)
+    </simpara>
+   </listitem>
+  </varlistentry>
   <varlistentry xml:id="constant.imagetype-unknown">
    <term>
     <constant>IMAGETYPE_UNKNOWN</constant>