File::getMemberProperties(): minor tweaks after readonly merge

Follow up on 3480

I realized after the merge that I had not published a few follow up comments. Addressing those here.

* The new `is_readonly` key still needed to be added to the documentation for the `File::getMemberProperties()` method.
* For consistency in how the method is tested, adding the `is_readonly` property to all other test expectations, which makes the `testPHP81NotReadonly` test case redundant.
* Adding two additional test cases. In particular I wanted to verify (and safeguard) that the retokenization of the `?` to `T_NULLABLE` after a `readonly` keyword would not be broken (which it partially was, but that has now been fixed in 3513).

src/Files/File.php

@@ -1759,6 +1759,7 @@ public function getMethodProperties($stackPtr)
      *    'scope'           => string,  // Public, private, or protected.
      *    'scope_specified' => boolean, // TRUE if the scope was explicitly specified.
      *    'is_static'       => boolean, // TRUE if the static keyword was found.
+     *    'is_readonly'     => boolean, // TRUE if the readonly keyword was found.
      *    'type'            => string,  // The type of the var (empty if no type specified).
      *    'type_token'      => integer, // The stack pointer to the start of the type
      *                                  // or FALSE if there is no type.