PHP 8.2 | AtomParser: explicitly declare all properties

Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.

There are a number of ways to mitigate this:
* If it's an accidental typo for a declared property: fix the typo.
* For known properties: declare them on the class.
* For unknown properties: add the magic `__get()`, `__set()` et al methods to the class or let the class extend `stdClass` which has highly optimized versions of these magic methods build in.
* For unknown _use of_ dynamic properties, the `#[AllowDynamicProperties]` attribute can be added to the class. The attribute will automatically be inherited by child classes.

In this case, the property added are explicitly referenced in this class, so fall in the "known property" category.

Refs:
* https://wiki.php.net/rfc/deprecate_dynamic_properties

src/wp-includes/atomlib.php

@@ -86,6 +86,10 @@ class AtomParser {
 
     var $feed;
     var $current;
+    var $map_attrs_func;
+    var $map_xmlns_func;
+    var $error;
+    var $content;
 
 	/**
 	 * PHP5 constructor.