Add support for true type

The null/false types were alread accepted previously, even though
they are only legal as standalone types since PHP 8.2.

Non-trivial backport from 1eba33d4124a248dd5c970fa7d6064d2e31b8635.

lib/PhpParser/ParserAbstract.php

@@ -664,6 +664,7 @@ abstract class ParserAbstract implements Parser
             'false'    => true,
             'mixed'    => true,
             'never'    => true,
+            'true'     => true,
         ];
 
         if (!$name->isUnqualified()) {

test/code/parser/stmt/function/nullFalseTrueTypes.test

@@ -0,0 +1,56 @@
+standalone null, false and true types
+-----
+<?php
+
+function test(): null {}
+function test(): false {}
+function test(): true {}
+-----
+!!php7
+array(
+    0: Stmt_Function(
+        attrGroups: array(
+        )
+        byRef: false
+        name: Identifier(
+            name: test
+        )
+        params: array(
+        )
+        returnType: Identifier(
+            name: null
+        )
+        stmts: array(
+        )
+    )
+    1: Stmt_Function(
+        attrGroups: array(
+        )
+        byRef: false
+        name: Identifier(
+            name: test
+        )
+        params: array(
+        )
+        returnType: Identifier(
+            name: false
+        )
+        stmts: array(
+        )
+    )
+    2: Stmt_Function(
+        attrGroups: array(
+        )
+        byRef: false
+        name: Identifier(
+            name: test
+        )
+        params: array(
+        )
+        returnType: Identifier(
+            name: true
+        )
+        stmts: array(
+        )
+    )
+)